EDIT: Added more stepwise explanation, hopefully it is clear enough to be reproduced
EDIT 2: It was brought to my attention that the proof does not work for single digits (shown by a clear example). That is solved not, and the conclusion is the same.
EDIT 3: Changed the example for the regular case, because the example actually wasn't a regular case
EDIT 4: Added a lot of MathJax. Hope I didn't miss anything
Gareth McCaughan answers questions 1-3 beautifully, here is my proof for q4 (is there an arbitrarily long sequence of number that can be written as M+rev(M)?):
The maximum length of a sequence we can find is 3
Proof:
Let us start with number $N_0$ which fulfills our rule, namely $N_0=M_0+rev(M_0)$
Where $rev(M)$ is the digit reverse of M: $rev(123)=321$
Let $N_0$ be an even number
Let $M_0$ be a number of $L_0$ digits
Let $p_0$ and $r_0$ be the first and last digit of $M_0$, such that:
$p_0$ and $r_0$ are digits, that is in the range ${0..9}$
$p00...00r <= M_0 <= p99...99r$
Let $N_{-2}$ equal $N - 2$, where also $N_{-2}$ can be written as $M_0+rev(M_0)$
Let the first and last digits of $M_{-2}$ be $p_{-2}$ and $r_{-2}$
If $N_0$ is an even number, so must be $N_{-2}$
Therefore, $p_{-2}+r_{-2}$ must be even, because that is the only way the last digit of $M_{-2}+rev(M_{-2})$ can be even and therefore $N_{-2}$ can be even
The last digit of $N_{-2}$ is the last digit of $p_{-2}+r_{-2}$. Which can be written as $p_{-2}+r_{-2} mod 10$
The same is true for $p_0$ and $r_0$ regarding the last digit of $N_0$
Since $N_{-2}$ is $2$ less than $N_0$, so must the last digit of $N_{-2}$ be $2$ less than $N_0$, and therefore must '$p_{-2}+r_{-2} mod 10$' be $2$ less than '$p_0+r_0 mod 10$'
An exception [case 1] occurs is when $p_0+r_0 mod 10 = 8$ and $p_{-2}+r_{-2} mod 10 = 0$
Special cases [case 2] occur when: $p_0+r_0 = 10 + p_{-2}+r_{-2} - 2$ or $10 + p_0+r_0 = p_{-2}+r_{-2} - 2$
Exception [case 3] is when number of digits $L_0$ is not equal to $L_{-2}$
Exception [case 4] is when $M$ and $rev(M)$ are single digits
I will cover these expections later.
For the regular case, where $p_0+r_0 = p_{-2}+r_{-2} + 2$ and [case 1, 2 and 3] do not apply, we can state:
$N_{-2}$ can be at most $p_{-2}99...99r_{-2} + r_{-2}99...99p_{-2}$
So $N_{-2}$ is at most $(p_{-2}+1)*10^L + (r_{-2}+1)*10^{L-1} - 20 + p_{-2} + r_{-2}$
Which can be written as: $(p_{-2} + r_{-2} + 2)*10^{L-1}$
We know that $p_0+r_0 = p_{-2}+r_{-2}+2$, so:
$N_{-2}$ is as most $(p_0 + r_0)*10^{L-1} + p_0 + r_0 - 2 - 20$
$N_0$ is at least $(p_0 + r_0)*10^{L-1} + p_0 + r_0$
For the more visual-minded, an example:
Given $L=4$: $N_{-2} <= 2994 + 4992$ [$p_{-2}=2,r_{-2}=4,p_0+r_0=6$]
Given $L=4$: $N_0 >= 2006 + 6002$ [$p_{-2}=2,r_{-2}=6,p_{-2}+r_{-2}=8$]
$N_{-2} <= (2+1 + 4+1)*10^{4-1} - 20 + 2 + 4$
$N_{-2} <= 8000 - 20 + 6$
$N_{-2} <= 7986$
$N_0 >= (2 + 6)*10^{4-1} + 2 + 6$
$N_0 >= 8008$
Whatever $p$ and $r$ we choose, $N_{-2}$ can never be more that $N_0-22$, so it cannot be $N_0-2$, which is a condradiction. Therefore, our initial statement that an even number $N_{-2}$ exists which equals $N_0 - 2$, where $N_0=M_0+rev(M_0)$ is false.
However, we didn't look at our exceptions yet.
What if $L_0$ does not equal $L_{-2}$ [case 3], which is the case in for example odd number $1300 + 0031 = 1331$ and $617 + 716 = 1333$
This can only occur if the number with smallest $L$ overflows to add an additional digit. This must be a leading $1$
Therefore, the number with the largest $L$ must not overflow
If that number does not overflow, its $p+r$ must equal $1$, since the leading digit equals $1$
If that is the case, then the last digit must be a $1$
This is an odd number. Therefore, this exception does not apply to our proof of the regular case and our proof holds
This same argument goes for [case 2].
The statement $p_0+r_0 = 10 + p_{-2}+r_{-2} - 2$ or $10 + p_0+r_0 = p_{-2}+r_{-2} - 2$
Has the same meaning as saying either $N_0$ or $N_{-2}$ is the result of digit overflow
This means that also the first digit will overflow
Therefore, this can only happen is the length of $L_0$ does not equal $L_{-2}$
As mentioned for [case 3], this can only results in odd numbers given our conditions
Therefore, the proof of our regular case still holds
And again, the same argument goes for [case 1]
The case where $p_0+r_0 mod 10 = 8$ and $p_{-2}+r_{-2} mod 10 = 0$ is just a special case of $p_0+r_0 = 10 + p_{-2}+r_{-2} - 2$
This only happens if the last digit overflows
Therefore, the first digit must overflow too and $L_0$ must not have the same length as $L_{-2}$
We already showed that if that is the case, not valid even number exists given our conditions
For [case 4], this does not work, so we have to do something different
Let's start by showing that this will work for any 2-digit number or larger, before going into the single digits.
For this, we go in from a different angle, but the proof is the same
Again, let $N_0$ and $N_{-2}$ be two even nubmers, $N_0=N_{-2}+2$, written as $N_0=M_0+rev(M_0)$ and $N_{-2}=M_{-2}+rev(M_{-2})$
If $M_{-2}=p_{-2}r_{-2}$, then $N_{-2}=p_{-2}r_{-2}+r_{-2}p_{-2}$
$N_{-2} = 10*(p_{-2}+r_{-2}) + p_{-2} + r_{-2}$
$N_{-2} = 11*p_{-2} + 11*r_{-2}$
And since we know $p_0 + r_0$ must equal $p_{-2} + r_{-2} + 2$:
$N_0 = 10*(p_{-2}+r_{-2}+2) + p_{-2} + r_{-2} + 2$
$N_0 = 11*(p-2) + 11*(r-2) + 22$
So $N_0$ is $22$ greater than $N_{-2}$, and can therefore not be $2$ greater than $N_{-2}$. This pair cannot exist.
It's not surprising, because there is no wiggle-room for additional $9$'s in the middle of the number.
For a 3-digit number this also holds:
The maximum value of $N_{-2}$ is $p_{-2}9r_{-2} + r_{-2}9p_{-2}$
$N_{-2} = 100*(p_{-2}+r_{-2}) + 2*90 + p_{-2} + r_{-2}$
$N_{-2} = 101*p_{-2} + 101*r_{-2} + 180$
$N_0 = p_00r_0 + r_00p_0$
$N_0 = 100*(p_0+r_0) + 0 + p_0 + r_0$
$N_0 = 100*(p_{-2}+r_{-2}+2) + p_{-2} + r_{-2} + 2$
$N_0 = 101*p_{-2} + 101*r_{-2} + 202$
So $N_0$ is at least $22$ greater than $N_{-2}$ and can therefore not be $2$ greater than $N_{-2}$
For single digits this does not hold. This is shown by $M_0=6$ and $M_{-2}=5$, leaving $N_0=12$ and $N_{-2}=10$
Any $N$ below $10$ cannot be odd as it must be constructed by adding 2 single digits, where $M=rev(M)$, so $N$ must be even
This means the in the range $10<=N<=21$ we have potential for more than 3 consecutive numbers
(Note than $N=20$ must be constructed from multi-digit numbers, so if it can be written as $M+rev(m)$, then $22$ cannot)
We know the even numbers in this range up to and including $18$ are possible $(10,12,14,16,18)$
We know that $11=10+01$
We know all odd number in this range must be constructed of multi-digit numbers
For numbers $13,15,17$ to be constructed of multi-digit numbers we cannot use digit overflow (because single digits required)
For number $19$ we cannot use digit overflow (has all $9$'s behing leading $1$)
For any number in the 10's constructed from multi-digit numbers, we need an $M$ with lagging/leading $0$ to leave a $1$ as first digit
the only 2-digit number that fulfills these rules is $10$
So in this range only odd number $10+01 = 11$ can be written as $M+rev(M)$
This gives us a nive bonus trio of $10, 11, 12$. But that's everything
Of course, we could have just checked every case op to $21$, but where's the fun in that!
To sum up, there are no pairs of even numbers greater than $20$ separated by $2$ that can both be written as $M+rev(M)$, because:
If $N_0$ exists that is an even number and can be written as $M+rev(M)$, the number $N_{-2}$ which is $2$ smaller than $N_0$ cannot be written as $M+rev(M)$.
This means that also $N_{+2}$ which is $2$ bigger than $N_0$ cannot be written as $M+rev(M)$, because if it were, then $N_0$ could not be written that way.
In the range below $20$, this proof does not work. However, we found out that the only odd number in this range that works is $11$. Therefore by similar argument no $N_0 = N_{+2}$ exists for odd numbers below $20$
Therefore, the largest sequence of consecutive numbers that can be written as $M+rev(M)$ is three, where the first and last are odd, and the middle one is even.
Their existence is shown by many examples.