#1
I'm having trouble making difficult chord changes (no anchor fingers or moveable chords) when playing eight note strumming. First strum after change often becomes partially muted, and the whole progression sounds very start&stop-ish. Any exercises/tips on making the flow tighter?
#2
Usual advice (assuming you have 4 fingers :-)  

Isolate two chords with the problem, and just make the change very slowly, and attend to every detail.  If you're getting muting, then you haven't completed the process of holding the new chord sufficiently before the strum takes effect.  You want all fingers to hit the new shape at the same time (unless there is the possibility to move some fingers early).
#4
Saesang Yes.  You can try gradual speed up.  (e.g. 5% change (maybe 2 or 3 times in 30 minutes) until your making mistakes, and then back off.  And also, try it where you know you have problems, briefly and then go back to slow and build up.  It'll come .. just the brain and fingers getting used to it, and eventually the fingers doing most of the work "unattended" by the brain as such.  Some aspects of technique just take a bit of patience, but mostly that's worth it!