Throughout this article, let $(P,\prec)$ be any countable, dense, and linearly ordered set without endpoints enumerated by $(p_n)_{n \ge 0}$.
Proposition 1: There exist an injective morphism $f: \mathbb Z \to P$ with the property that
$\tag 1 f(0) = p_0 \text{ and for every } p \in P \text{ there exist } b \in \mathbb N \text{ such that } f(-b) \le p \le f(b)$.
Proof
We will define a sequence of injective morphisms,
$\tag 2 f_k:[-k,+k] \to P \text{ such that }$
$\quad \quad \quad \text{ for every } p_n \text{ with } n \le k,\; p_n \text{ lies between } f_k(-k) \text{ and } f_k(+k)$
and each $f_{k+1}$ is an extension of $f_{k}$.
We start with $f_0(0) = p_0$. Assume that $\text{(1)}$ holds for a fixed $k$. If $f_k(-k) \le p_{k+1} \le f_k(+k)$, we just choose any extension. If, say, $p_{k+1} \gt f_k(k)$, we define $f_{k+1}(k+1) = p_{k+1}$ and select an extension defining $f_{k+1}(-(k+1))$. A similar argument extends $f_k$ to $f_{k+1}$ satisfying $\text{(1)}$ when $p_{k+1} \lt f_k(-k)$.
The union of all the $f_k$ is the function $f$ we are seeking. $\quad \blacksquare$
Let $U = \{m2^{-n} \; | \; \text{ with } m \in \mathbb Z \text{ and } n \in \mathbb N\}$. It is easy to see that the set $U$ is a countable, dense, and linearly ordered without endpoints under the usual $\lt$ relation.
Observe that by setting $U_k = \{m2^{-k} \; | \; m \in \mathbb Z\} $ we can assert that $U_0 = \mathbb Z$, $\,U_k \subset U_{k+1}$ and that
$\tag 3 U = \bigcup_{k \ge 0} \, U_k$
Set $F_0 = f$, where $f: U_0 \to P$ is any injective morphism satisfying $\text{(1)}$.
Lemma: If $F_k : U_k \to P$ is an injective morphism that extends $F_0$ such that the elements $(p_n)_{0 \le n \le k}$ are all in the image of $F_k$, then we can extend $F_k$ to an injective morphsim $F_{k +1}: U_{k+1} \to P$ such that $p_{k+1} \in F_{k+1}(U_{K+1})$.
Proof
Note first that the set $U ∖ U_k$ can be partitioned into 'open intervals', and the same can be said about $P ∖ F_k(U_k)$. Also, between any two consecutive points in $U_k$ we can find a midpoint of the form
$\quad \beta_{(m,k+1)} = \frac{m}{2^{k}}+\frac{1}{2^{k+1}}$
and these are precisely the points in $U_{k+1}$ that are not in $U_{k}$. For each one of these points, we can choose an element $q_{(m,k+1)}$ in $P$ between $F_k(\frac{m}{2^{k}})$ and $F_k(\frac{m+1}{2^{k}})$ and then map $ \beta_{(m,k+1)} \mapsto q_{(m,k+1)}$.
With this in mind, we see how to extend $F_k$ if $p_{k+1}$ is already in the range of $F_k$. But when $p_{k+1} \notin F_k(U_k)$, it is in exactly one 'open interval' of $P ∖ F_k(U_k)$ and that interval contains an element $q_{(m,k+1)}$. So all we have to do to take care of business by swapping out our selected $q_{(m,k+1)}$ and replacing it with $p_{k+1}$,
$\quad \beta_{(m,k+1)} \mapsto p_{k+1}$
and thereby ensure $p_k$ is in the range of $F_k$. $ \quad \blacksquare$
Proposition 2: There exist an isomorphism between $U$ and $P$.
Proof
Our recursion starts out with $F_0$ defined on $\mathbb Z$ and satisfying $\text{(1)}$ so that $F_0(0) = p_0$. The lemma allows us to recursively extend to a function $F_k$ that is an injective morphism that contains $p_j$ for $j \le k$. By applying the lemma and taking the union of all the $F_k$ (their graphs), we get an isomorphism $F$ between $U$ and $P$.$\quad \blacksquare$
The following is an immediate consequence of proposition 2.
Theorem 3: Let $P$ and $Q$ be any two countable, dense, and linearly ordered sets without endpoints. Then they must be isomorphic.
Note: We do not have to use the axiom in choice to prove the lemma. To 'crank out' $q_{(m,k+1)}$, we can define it to be correspond to $p_j$ where $j$ is the smallest natural number satisfying $F_k(\frac{m}{2^{k}}) \lt p_j \lt F_k(\frac{m+1}{2^{k}})$.
The same goes for proposition 1. When we state 'choose any extension' we can put the enumeration $p_n$ to 'double use' and make it concrete.
So we can arrive at Theorem 4 without using the axiom of choice. The back-and-forth algorithm doesn't use it, so this is not surprising; see Back and forth and the axiom of choice.
A good exercise is to carefully prove proposition 1 without using the axiom of choice.