1. In 895 the Bohemia / Moravia (including Silesia at that time) princes came to Regensburg of Frankish and Holy Roman Empire and pledged allegiance to German emperor Arnulf of Carinthia .
2. 622-662 first Slavs in Europe are the Moravians under Frankish ruler Samo.
3. Mieszko I was first duke of a Poland seperat from Czech (formerly Czech and Lech)Mieszko I and son Boleslaw I Chrobry (as well as later Polish rulers pledged allegiance to emperor in turn for receiving land on loan (landlien) to rule over.
4.The Polish Piast rulers died out in 1370. (Imperial law, when a ruling house dies out , the land reverts back to the empire.
5. In circa 1537 Brandenburg Hohenzollernrulers made an agreement with the Silesian German Piasts, that Brandenburg would inherit Silesia rule. Emperor did not permit it, because the emperor (Maximilian I) had made an agreement in 1515 with Polish and Hungary ruler .Maximilian adopted Luis of Hungary.
6. Crossen , Or Krossen district (Schwiebus referred to in above website as 1815) including Zuellichau came under Brandenburg rule in 1472.
Are you sure about etymology of name Silesia ? It seems too similar to its Polish name Śl&aogonek;sk which is of clearly different etymology (from Mount Śl&eogonek'&zdotabove;a)
There are many other geographical Polish names that were latinized with ending -ia, and there was either a short vocative e/i vowel (vocal yer) between first two consonants or second consonant was a vocal l, which sounds like el/il, so s(short i)l(something that sounds like on)sk(short e)/s(short i)l(something that sounds like nasal e)(hard vocative z)- -> silesia seems very likely.
Sorry, but your etymology sounds like some German propaganda. --Taw