I would not put a double main. The track to scenery ration would go to high, the curves diameters are awkward to make interesting and overall would take away from your goal.
You will find that the accessories and buildings take a lot of real estate. Just start placing them on the table to see for yourself.
I like the full trestle set to put on show on that long wall. It's rather boring if you don't.
Post new topics on tuning these accessories. There is a wealth of experience and knowledge on how to make them work reliably. There is also, Olsen's, which has a library of service documents that sometimes have tuning and modification tips. Here is the 128 newsstand. The string coming off is due to too high of a voltage. You'll find that each accessory has a sweet spot for voltage. Many times you can videos on YouTube of them working.
For the newer accessories, viewing the Lionel parts diagram and getting assistance from forum members definitely helps.
Trees, you cannot have enough trees. Bottle brush trees work to get going and are relatively inexpensive.
Don't lose sight of your goal -get the heirloom trains up and running - keep it simple to accomplish it. You and the boys will want to build another one together.
here's the latest version of the layout. Added another siding to the O layout using one of the switches that you acquired.
The Rico station silhouetted against the window light would look nice. it is both freight and passenger.