Hi David. Great to hear you're thinking about what you want to do and what you are most passionate about. I'm fairly new at this myself, but I'll answer as best I can.
1. Studios can do studio-sized projects, and things that a freelancer might not be able to handle. However, the freelancer is most likely cheaper and can give more personal service. I definitely learned that you need to be careful how much you take on at once. Having 2-3 clients calling and emailing you during the day really hurts productivity. It's tough to say no to projects, but it's better than taking on too much and disappointing all clients. I'm more than happy to just take on small projects right now that fit the "lone wolf" category, and these definitely exist.
2. No idea, but I think you could specialize. I think it depends on the scope. A nice residential exterior or an office interior might be very managable for a freelancer. A huge shopping mall for an interior, or an airport for an exterior might be too large. I think there is a market for both exteriors and interiors, and if your quality is excellent you will be in demand.
3. Again, all I can speak about is what's worked for me. I have a background of almost 10 years in Max, and it really can do everything. I think it's the perfect tool for the freelancer. And you have access to great plugins too. Vray is an absolute must-have if you want to make money off your renderings. The quality and ease of use is so much higher than what I've experienced with Mental Ray. I actually was thinking recently on an interior I was doing, how easy it was to get smooth and correct lighting. I couldn't recall the last time I had to battle noise or blotchiness since switching to Vray. In Mental Ray this was just part of the process, battling the engine for quality. It just simply works, and if I could change anything in my career I would have learned it sooner.
I don't know your level of comfort with architecture in particular, but what I did was get a certificate from a community college in architectural drafting. This gave me experience with AutoCAD, Revit, Sketchup and Solidworks. I think it helped to put me in the architects mindset, as opposed to just a Max guy who can render buildings.
4. Depends. I have some clients who just want images, and some who like to open the files and tinker for themselves (they also know Max a bit.) Some others literally just want models in OBJ format. So I would say it all depends. I've had to turn jobs down that ask for Maya experience, and that's okay. I know Maya people who have turned down Max jobs. It's better to be excellent at one major software I think, than to spend time learning them all. That's an impossible chase anyways, not enough time in the day.