I have little doubt it's a software issue. I've experienced my own frustrations with crashing when there's too much in the viewport. It can choke up if there's too much stuff showing at once, but that seems to be the only time it chokes up for me. And for me, non-shaded wireframe mode really helps, as OpenGL/Software has much less work to do. What OS are you using? I'm still on 7, so maybe the oldness of my OS is helping my program's stability, as weird and backward as that sounds.
Ultimately I'd like to see your source files in max to compare with our other map files, as I don't experience such performance problems as you're describing when I load those. It could also be chalked up to an enormous size difference between stock maps and Siege. I know you said it's split into four files (none of the stock maps require this afaik); what filesizes are you looking at now? Have you painted trees/grass/rocks yet? Your last few screenshots look really pretty with all the ivy, but too much of that stuff can also eat performance, so striking a balance is paramount.