Witness intimidation

Question

After solving this puzzle, I received an envelope in the mail. I opened the envelope, only to find three letters inside. The first one read:

I know who you are
I'm sending my greeting
Investigate further
And your lives will be fleeting

Astonished at the blatant threat, I quickly scanned over the second letter:

Unfortunately, I couldn't make heads or tails out of it, until I took a look at the third letter:

After a brief moment, I assembled a message and was instantly horrified!

What greeting did the letters contain?

Hint:

• One can grow itself
• One's namesake for constant
• One's place for logo
• One's connective
• One describes an individual
• One's noisy
• One follows Nice for a meme? (4 4)
• One's a gate
• One may be gassy
• One's to end
• One's a tree?
• One tells time
• One's expression
• One.
• One's what melted
• One's holey tool
• One Direction?

Hint #2:

Each picture helped to make heads or tails of the corresponding word

Hint #3 (strong hint):

Placing words on the grid (top to bottom or left to right), the images anchor either the beginning or ending of the corresponding word.

Hint #4:

Some numbers are red herrings

Answer 1

The main puzzle mechanism has already been worked out thanks to @LukeBickell and @MOehm - go upvote their answers (@MOehm - feel free to add this into your solution so we have a complete one; @Avi - give the checkmark to that one!). All that remains is the final message, which I believe is:

HELLO PUZZLERS!

How?

Look only at the words relating to the symbols explicitly mentioned in the third letter received by the OP (ignore all the others - these are red herrings). For each of these, note the numbers written in the corners of their symbol in the first grid and examine the corresponding index position(s) in the word to get:

pERSon
bUZZer
chipotLe
pLanck
pEriod
smiLe
Pot
tHaw
dOwn

Altogether this gives us: ERSUZZLLELPHO. Finally, note the thus-far-unused anagram tag. This letter sequence anagrams to 'HELLO PUZZLERS!' Your mysterious correspondent appears to know who we are!

Answer 2

Partial answer, building on Luke Bickell's.

The things in the grid are:

One can grow itself           POT¹
One's namesake for constant   PLANCK
One's place for logo          CHIPOTLE¹
One's connective              OR
One describes an individual   PERSON¹
One's noisy                   BUZZER²
One follows Nice for a meme   BOAT
One's a gate                  NOR
One may be gassy              STAR
One's to end                  ABOLISH
One's a tree?                 OAK¹
One tells time                ASTROLABE²
One's expression              SMILE
One.                          PERIOD¹
One's what melted             THAW
One's holey tool              AWL
One Direction?                DOWN

Answers marked (1) were found by Luke Bickell; answers marked (2) were found by Stiv. (I had found the other answers even if they appear in Luke's answer, but I was wrong about the pot and the period and would never have got Chipotle.)

And the grid looks like this:

     · · P e r s o N · B
     A w L · · · · o · u
     s · a · S t A r · z
     t · n · m · b · · z
     r · c h i p o t l E
     O a K · l · l · · r
     l · · P E r i o d ·
     a · · o · · s · o R
     B o a T · T h a w ·
     e · · · · · · · N ·

Here, the words are placed in crossword-style, so that they cross, but don't run into each other when going in the same direction. The first or last letter of a word coincides with the respective symbol in the grid. These "anchors" are written in capital letters in the grid above.

And now?

I don't know. There are nine symols on the third sheet and the numbers in the grid go from one to nine. What do the positions of the numbers mean? The numbers cannot be letter position, because in some places, these positions exceed the length of the answers. But I note that the first three things from the third sheet are Person, bUZZer and chipotLE, which could conveniently spell something, in the case of buzzer even with the letters indicated by the numbers.

Answer 3

While I have no idea how to use the grid and numbers, I at least have a number of guesses for the words, which may help someone else.

• One can grow itself --> POT (you can grow pot (marijuana) in a pot)
• One's namesake for constant --> PLANCK'S constant (is represented by a cursive h)
• One's place for logo --> CHIPOTLE PEPPER (is the logo for Chipotle restaurant)
• One's connective --> ?
• One describes an individual --> PERSON ?
• One's noisy --> ?
• One follows Nice for a meme? (4 4) --> ?
• One's a gate --> NOR (a logic gate)
• One may be gassy --> STAR (are mostly comprised of gas)
• One's to end --> CANCEL STOP (common symbol)
• One's a tree? --> (PROFESSOR) OAK (8-bit character from Pokemon)
• One tells time --> CLOCK/WATCH
• One's expression --> SMILE
• One. --> PERIOD
• One's what melted --> WATER (ice to water) ICE (water doesn't melt, ice does)
• One's holey tool --> AWL (hole making tool)
• One Direction? --> DOWN (arrow pointing down)

The remaining three I don't have guesses for are the boat, the bowl/semicircle, and the V.