Prove number of nodes in heap
Interesting proposition.

Moreover, the trees in a collection can have height 1,3,4,8 as an example. It is not limited to increasing by 1.

How could 1,3,4,8 be an example? If you set $i=0$ in &quot;there exist at least $i + 1$ x-heaps of $height \leq i$, where $0\leq i \leq k$&quot;, you know there exist at least 1 X-heap of height less than or equal to 0. That means $X_0$ must be 0. In fact, the height will be increased by 0 or 1 every time.
Reformulation
Since an x-heap is completely determined by its height, let us introduce almost-natural sequences, which comes from a collection of x-heaps in the question.
An almost-natural sequence is a non-decreasing sequence of integers $0=h_0, h_1,\dots,h_k$, where $k\ge0$, such that, for all $0\le i\le h_k$,

at least $i + 1$ of them are not greater than $i$
at most $i + 2$ of them are not greater than $i$.

Characterization of almost-natural sequences. An almost-natural sequence must be one of the following.

$0, 1, \cdots, k-1, k$, i.e., the first $k+1$ natural numbers.
$0, 1, \cdots, m-1, m, m, m+1, \cdots, k-2, k-1$, i.e., the first $k$ natural numbers with one of them, $m$ repeated once, where $0\le m\le k-1$.

Corollary. $2^k\le\sum_{i=0}^k2^{h_k}\le2^{k+1}-1$ for an almost-natural sequence $h_0,h_1,\dots,h_k$.
Exercises
Here are a few related exercises.
Exercise 1. Prove the characterization of almost-natural sequences.
Here is a hint. Does a given almost-natural sequence contain a repeated number? If yes, prove that number appears exactly twice and no other number is repeated.
Exercise 2. Prove the corollary.
Exercise 3. Let a d-natural sequence be a non-decreasing sequence of $k+1$ integers $0=h_0, h_1,\dots,h_k$ such that at least $i+1$ and at most $i+1+d$ of them are not greater than $i$ for all $0\le i\le h_k$. Determine the lower bound and upper bound of $\sum_{i=0}^k{h_k}$ and $\sum_{i=0}^k2^{h_k}\,.$