I didn't realize that your walls were closed. You could do it from the bedroom side, you just have to do a more careful patch job since you will be able to see the drywall. Insulation isn't typcially added around pipes, so it isn't something a contractor would automatically do. We chose it put insulation between all of our rooms to help reduce noise. Also, you could access the pipes from the inside of your cabinet if you cut the back of the out. The thought does make me cringe, but there again, how often does anyone get down on the floor and look at the back /inside of the cabinet?

Yeah I just opened the Lazy Susan last night and realized the toe-kick area has a bunch of walls I would have to cut thru to route the DW drain pipe. I'm thinking about just going around the back now to avoid multiple cuts.. I just don't know whether to make a gradual decline from the DW to the garbage disposal (1), or just route it low around the lazy susan and then make a "high point" right before entering the garbage disposal (2)..? Does it matter?

Plumbing new drains and vents can be complicated, but in this case it seems you are just replacing your old shower pan and drain with a new curbless design. You are not really changing the basic plumbing design; you are just trying to get the drain to align. I assume you have fully researched the design for a curbless shower, since that is more complicated to implement than your plumbing changes. In most cases its easier to use a pre-sloped pan covered with a waterproof membrane by Schluter or other good company. They recommend also using waterproofing membrane under the bathroom floor for several feet around the shower, since its hard to predict how much water will end up on the floor when you have a curbless design. This company has a low profile P-trap, but I'm not sure it will be low enough for what you need. https://pvcpipesupplies.com/1-1-2-p-trap-low-profile-dwv-fitting-d706-016.html If the vertical pipe in the wall is accessible and made out of PVC or ABS, it is a relatively simple job to cut it and lower the drain connection. If you are motivated enough to break up concrete, cutting into and splicing a drain pipe should not be that hard for you. Bruce

If there's a hollow core in the column your best bet would be to pop the cap off, run 1/2" pvc to the bottom and elbow it out through the side in an inconspicuous spot. Drill out the cap and set it back on, trim the pipe, and set your planter. Only your hairdresser needs to know... Otherwise, I've never seen what you're describing. I'd get a pan with the highest side walls you can get away with, drill it out for a bulkhead fitting, run 1/2 pvc and paint it to blend with the column using Krylon or something similar.