No technical reason that i can think of for not dropping the waste trap other then taking up loads of room

perfect solution is to take the waste pipe along the wall if possible with out a clearer picture it is hard to say.

If putting up Wall units a good way is to put a batten along the wall at the hieght of the bottom of the wall units and then sit them on that and then fix them back to the wall Rather then some of the crappy solutions that come with sme wall units.