I think that the reason new military tactics has to work the way it
does,is that you are discarding the card to give yourself 3 extra
military for that turn - not to pay for a world.This can actually
matter when you combo it with improved logistics,as you can then use
it to help you conquer two worlds.
In the case of doomed world,if it didn't work that way
you could sacrifice it when you had no valid worlds to settle in order
to avoid having the minus one point,and you aren't supposed to be able
to do that.Or else,you might find yourself unable to ever complete
your move -depending on how the system worked.
On the subject of the system
accepting erroneous moves,I have accidentally ticked too many cards to
pay for something.The system accepted the cards in tableau order [top
left to right,followed by bottom left to right].I wound up paying the
correct number of cards,but including a card I had intended to
keep.That is a bug - it should have refused my instruction and made me
select the correct number of cards.