1. Your running the latest officially support version of Android for your tab.
2. There is an unofficial version for GSM tabs, but I'm not sure if you ned to have a T-Mobile specific version or not so I would read and proceed with caution. [ CLICK HERE
3. Any time you install something not officially supports, you'll need to read and be aware of al possible issues. I'm not actually positive the link I provided is even supported on the T-Mobile unit. However it does state GSM versions. I would post the question about whether or not that Gingerbread update is T-Mobile Galaxy Tab supported. If that info is not already listed in the thread.
4. See above.
David, remember you're running one of the first Android tablets to officially be sold. In a sense it was half-baked. Samsung and a few others wanted to quickly compete with Apple's iPad. They got tired of waiting for Google to release a tablet ready Android OS, so they tweaked an existing version of Android use on phones then released their tablet.
Later Google developed what's known as Android 3.0, Honeycomb. Then HTC, Samsung, Motorola all rushed to have a proper tablet. Because of the interim release of the tablet you received, don't expect Honeycomb (Android 3.0) or even Gingerbread support. Neither the carrier nor the manufacturer wants to flat out say, it's not going to happen our that could cost them in future. You might not consider buying a Samsung product going forward. Easier to just place the blame.
In place of an official upgrade, XDA is one of the biggest mobile hacking websites out. They will rip operating systems and make them work with various devices. Hence the link I provided you.