Here the answers of your questions.
1.)Possible I think. You can hack exeDSP for to do that but this requires some work. And It's not easy at first look too.
2.)Yes 940x540 due limitation of SDL software implementation of this TV. Since Movies doesn't use SDL interface, they could show up to 1920x1080p

3.)Yes, possible if you write browser.But I don't know id there is another reliable solution instead of NetSurf. I guess any SDL browser that has JavaScript support run on TV with little work..
4.)It changes between models. A series have SH4 CPU, B series (which SamyGO currently relies on) uses ARM v6 CPU, C series are using MIPS. LExxB650 has 512MB RAM,1GB Flash

5.)B650 is good. Since you are planning to use internal components with your specialized software, you better to look spec sheets for this TV models.
* You have to know that you cannot install some linux distro on it and work this TV as native linux box easily.