1. Depends, do you have an iPhone 3G, iPhone 3GS, or iPhone 4? I have an iPhone 4 on 01.59.00 and it does have the lost signal/1 bar problem. This problem has been lingering around since iOS 4.1. I also used redsn0w to update my iPhone 4 to iOS 5.0.1 while preserving the baseband. You have to create the custom ipsw first, put your phone into DFU mode, and then shift restore in iTunes.
2. I've never had to downgrade my iPhone, but I don't think iOS 5.1b3 should be used for daily use, but SHSH blobs should allow you to downgrade.
3. Like I said, the no service problem has been around since iOS 4.1, the only thing I do when my phone has no service in a place that has service is to keep rebooting to no end until ultrasn0w finally kicks in and gets service.