It is better to make a frame for the mountain and skin cover it rather than going for a solid mountain. In this case weight can reach really high levels
Yep, the best place to get answer on your dio is the Diorama Forum.
I agree with making the mountain hollow. I have worked on several train layouts and that is the best way to go.
I cut a few pieces of wood to support a screen. I then stuff crumpled paper under the screen to give some support. You can then dip paper towels into plaster of paris (or similar) and lay over the screen. It’s a good idea to color the plaster with brown or green food coloring. This way, if it does chip, it doesn’t show as a bright white spot. If you can, try to use the paper towels you find at a hrdware store. They tend to be a bit rougher and more paper-like (not like soft paper towels)
Once dry, you can add more plaster to give it some thickness and add grass, scenery and such.
Welcome to the forum, always good to meet new people. For your mountains and terrain I would suggest you picking up the Osprey book called “Terrain Modeling”. Its been a real great help on my terrain. It covers everything you need to know about making many different forsm of terrain. From small pebbles to great big mountains, from streams and mudholes to oceans for your ships. The book cost me $30 , but its well worth it. check out www.ospreypublishing.com