I have a seeding that the top died back on. Is it true that this tree will not reach the node count now or will auxillary branches if trained upward as the main trunk eventually mature?

On the same topic. Is it true that wood taken from the lower branches of an immature seedling and budded or grafted onto good rootstock will also never mature?

In my understanding, those could mature too but their maturing needs more time.

Correct, both will mature & fruit over time.

Ok great, I wanted to know whether to give up on the seedling or not and this is good information to have regardless.