From my small amount of experience with layouts, figure whatever you think you will need for space and double it. Everything I have done so far that sounded like plenty of room to start with has been too small and outgrown very quickly. 26 x 19 sounds adequate, but I am sure it will shrink up fast when you start building your layout. I have only modern diesels, no articulated, but everyone always says use the widest curves possible.
I think an around the room will get you more track on your layout for the space used.
No duck unders, lift out, swing out or anything but having to duck. As you say getting older only makes it worse.
I think I would prefer rectangle, possibility for longer straight runs.
As many sidings as possible, but I think you need to also allow plenty of space for buildings, scenery, roads, etc. I have a tendency to always try to use too much track which is not good.
I would say at least 18" between wall and layout for access. If it will be used for operation then I would say probably 24"-30" for proper passage while operating. (Along with not being able to get under the layout anymore, I haven't gotten any smaller with age either)
LED lighting is coming down in price and I think a lot of folks are starting to use those or thinking about converting theirs to LED. Less heat, saves energy, longer bulb life. Dave Hikel suggested to me a while back that 5000k was very good to use in a train room. I tried that with the fluorescents I have and he was exactly correct, I really like the light from them with the 5000k, much better than the 3000k & 6000k I had tried before. This is a personal taste item though, so you may not like that as well as I did?
For electronics, if you are 3 rail and have both MTH PS2-PS3 and Lionel TMCC or Legacy I would get both a Legacy and DCS system so you can access the best features of both manufacturers products. If you are 2 rail I would get a good DCC system. I'm 3 rail so don't know much about DCC.
I have only been back in the hobby for about 3 years or so and I'm by no means an expert, this are just things I have learned here from reading on the OGR forum. Hopefully others with greater knowledge will be along to add more information for you.