If a treant (with 2 natural attacks - slams) elects to use only one natural attack (and no other attacks) in a combat round, does the treant add 1.5x the strength bonus or still use only 1x the strength bonus?
Still only 1x. Only some creatures that only have a single natural attack (like a wolf or something) get 1.5x. Thats an exception, not all creatures that only have a single natural attack get 1.5x even if they only have one attack. You should look at each creature's stat block to verify. If a treant, a creature with 2 natural attacks, chose only to make one, it wouldn't get that exception because it doesn't say it does.