I really need a sapling from a single tree

Solution 1:

A tree with all leaves cut off cannot regrow its leaves. In fact any leaves that are removed will not be regrown. Also bonemeal will only work on saplings, not on wood nor logs.

The only way to get another sapling is to destroy your second tree. There is a very high chance that your second tree will drop another sapling, and if it doesn't you will be stuck and lose anyways so it's better to find out sooner than later. Just be sure you make a dirt platform surrounding the tree to catch any items that drop as you destroy it - at least two blocks extending out each side from the tree's leaves. Losing one sapling could end up being gg.

Solution 2:

No, tree growth is a one time thing, from a sapling. It will never regrow its leaves, and there is no such thing as a tree stump. At that point it's simply a log block sitting on the ground.