You are right with almost everything except the 3G! I used several 3G phones from T-Mobile USA here in Germany they all worked fine on 3G! Its the other way were it doesnt work T-Mobile.de phone in USA!
With a T-Mobile germany SIM you don?t even need to unlock your phone cause it works just fine with it only for Vodafone, Base or O2 you need to unlock it.
By the end of this year the whole T-Mobile germany 3G network will be upgraded to 42Mbps, but keep in mind here it is not named 4G only LTE is named 4G here!
Vodafone offers a lil bit better coverage but the data speeds are a bit lower. O2 has also quite a good network.
Both T-Mobile and Vodafone have launched their LTE networks but they only use it as home internet yet.