so, put 'em on the ceiling since you want to move furniture --
dirt from the floor not so likely to fall up into the outlet--
one puts supplies at a wall opening since that is the biggest loss point --
windows have the biggest gain
'washing' exterior walls with conditioned air blocks heat transfer --
think of the air curtain doors [like at Costco]
think returns in ea room, not bath
just don't have supply within a few ft of return --
with good duct | register design, hi | lo placement of either will make little difference [not measureable]
having a good installation is important
attic over room?
your problem is HUMIDITY!
having an oversized unit will aggravate the problem --
you need to state your desired indoor temps.
you need to state that the % RH will be 35- 55.
that all rooms will be within 3F just after unit shutdown.
you have 'tightened' the envelope?
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!