What is the difference between strýci and strýcové?
No difference here. "strýci" can also be dative singular, but not here.
Why not “Matěj a SVEHO bratr...”
Because "svého" refers to the subject (it means exactly that: "the subject's") so it cannot be a part of the subject phrase.
This one also doesn't fit the days and week thematic
So what?