The solution for my issue #2 with title skipping when playing back via my wifi connection seems to be connected with my FritzBox Router > FritzBox Repeater setup. I followed the steps described in Titles Skipping constantly [resolved - deselected the Package Accelerator in Fritz Box] - #28 by Ole_Johannsen1 (disabling Package Acceleration in the repeater’s configuration), and so far the effect has not returned.
I am currently playing back albums where the effect was very noticeable in the past.
This is not yet a final confirmation, because the skipping problem showed up irregularly, and it needs some long-term testing. But if after a week of listening it does not re-appear, I am quite confident that it may be the solution to my major issue 
EDIT: After a lengthy stretch of listening to demanding albums/tracks that almost always had issues with skipping and drop-outs, I am fairly confident that this fixed my wifi issue.
Looking forward to the next product update for the fix concerning my wired setup. Thank you, @benjamin , for your support 