1.) Turn off your 3G/HSPA+, and you will get EDGE, which uses 1900 band. That should give you a good indiaction. But generally yes, HSPA+ over 1900 will have better building penetration than AWS.
2.) AWS HSPA+ shouldn't be affected coverage wise even. Bandwidth wise i'm unsure right now since Tmo WILL be adding LTE over AWS. Tmo currently uses (depending on location) 10x10(20MHz) for DC-HSPA+ (42 mbps) and 5x5(10MHz) for HSPA+ (21 mbps). If Tmo has enough spectrum in a market like my city Seattle (no less than 40MHz), Tmo will use 10x10(20MHz) DC-HSPA+ AND 10x10(20MHz) LTE. But I believe Tmo will start LTE at 5x5 in most markets initially. More details should come soon.
3.) Maybe. Tmo will have to COMPLETELY shut down their GSM/2G network so they can fully use 1900 for DC-HSPA+. That'll give them some breathing room for LTE over AWS since Tmo can shrink HSPA+ over AWS and not be penalized for it, and increase LTE over AWS in areas that were limited to only 5x5(10MHz). Remember, Tmo has an average of 20MHz of AWS in alot of markets. Some more than 20MHz.
Hope that helps you out a bit.