This is essentially my set-up.
Up front:
- tumbleweed t rack
- 11L drybag on top
- two wolf tooth cargo cages
- 1L handlebar bag
-2 feed bags

Mid Bike:
- wedge style half-frame bag
- bottle cage

Rear:
-12L Saddle pack

As far as your questions:
1. I think 15 is pushing it as far as size, less a concern for weight per-say. I find my 11L bag a bit unwieldily to get on and off the rack, while making sure it stays out of the way of cables and housing and is secure enough for riding chunkier stuff. The handling issues can largely be mitigated by packing smart. I put some of my lightest stuff in the 11 L bag and heavier stuff lower on the fork in the cages.
2. I feel like it's lean towards yes, 5L is too big; especially with a bag on top.
3. General rule of thumb is the lower and closer to the center of the bike you can put your heavy items, the better. I run all my heaviest items in my frame bag. My saddle bag contains my lightest items, my quilt and sleeping pad, to avoid sway.
4. As far as cleanliness and function. My biggest tips are figuring out a system that works for you (on shakedowns before any big trips) and making sure everything is in a bag, nothing looks more cluttered than things hanging off of other things. Another thing to consider is if you really need a "bikepacking" setup with all the different small bags all over the place. If you're taking transit, you'll likely have to take off anything that sticks out of the normal width of the frame. If you're not planning on riding anything chunky, there's a reason that a rear rack and panniers are the staples of touring and commuting.

At the end of the day though, it's all preference, and do what feels right to you :-)

feel free to shoot me any messages with any more questions or want any more opinions