1) what's the advantage of swapping in the mz3/mcoupe rear subframe? where does one put the jack stands underneath the rear end when one is replacing the rear subframe itself (as that is where i would normally put them)?
The main advantage is ease. Seven bolts hold that entire assembly in your car. All that has to be done is modify the brake lines because of the differences in the M calipers and the standard ti calipers. Your gains include: Bigger diff with LSD, cooling finned cover for diff, heavier rear suspension components, big M3 brakes, bigger half shafts... The jacks go on those black 'pucks' just ahead of the rear wheels. These are the lifting points, and this is their purpose in life. (if they are missing, they are cheap at the dealer. Don't support the car without the pucks! You will damage the underside of the car.)
2) with the e30 3.73 LSD, would the mz3/mcoupe driveshafts (complete, unmodified) work with the stock 318ti trailing arms, hubs, etc? are they the correct length?
No idea. See answer to question #1. You are making more work for yourself, and additional cost in the long run. PLUS, you will end up with a cobbled together rear suspension that uses the weaker trailing arms. and, FYI, this is not just to handle more power. Stiffer sway bars will rip the stock trailing arms to pieces if left alone long enough. Ask Steve...
3) what other modifications (brakes, etc) are needed if one replaces the stock trailing arms with those from the mz3? i believe i remember something about caliper location, brake lines, can't remember what else being different.
See answer to question #1. Now you see why this is the smart swap. The trailing arm alone cannot be swapped onto the stock ti suspension member. I think the bushings are larger on the M parts (anyone correct me if I'm wrong on this), and I think it would be a bad idea, because you transfer the weak link to the stock ti part. It is also possible the angle of attachment, although unlikely, is different as well.
4) regarding the mz3 trailing arms, do they also only have those lame tabs to which the anti-sway bar end-links attach or did they actually strengthen/redesign those? be nice to know i won't STILL need to weld in reinforcements on the mz3 trailing arms if i put those in.
The arm is larger, and has thicker walls. This makes it heavier, but with the heiver motor, it balances nicely. The mount is in the same location. The intermediate links between the bar and arm have different P/N's, and are assumed to be stronger as well.
5) if i don't or can't use the mz3/mcoupe driveshafts (too long, won't match to hub, whatever), what is the next best option? can i use my stock 318ti driveshafts (migrating my flanges from the 3.45 diff) or do i need to use e30 325 driveshafts or what?
See answer to question #1. SEVEN BOLTS. See how easy it is??? I would be suprised if a bone yard would want to sell it any other way than a complete assembly. (but you may find one less diff, but what good is that???)
6) where is a good source of used/rebuilt diffs/driveshafts/mz3 subframe/trailing arms, etc?
Check the back of Bimmer magazine. There are quite a few who advertise there. Call them all for quotation (don't forget shipping charges) and make the move!!
guess that's it for now. thanks in advance for all the help/info/opinions
You're welcome!
upgrading to e30 3.73 LSD + mz3 sub/trailing arms? 
Hybrid Mode
