As for the first question, no, you cannot discard them for
The Trees are strong if you already discarded them. You cannot discard minions from your discard pile, otherwise you cold create an infinite loop with those. So you just have to decide if you wanna discard them with
The Trees are strong or with their own ability, which doesn't make much a difference for me (at this example).
Same for
Isengard Builder. Once he's in the discard pile, you cannot discard cards from there. They are already discarded. It's like if you wanna kill a dead person. Simply doesn't work.