I've gone thru my book and it's brimming with info, here's what I could find though I'm not sure I can offer more than the previous forum, they all know this stuff off the top of their heads! lol
Some of this stuff has been explained already in your post but I'm just typing out what my book says.
1. In ref to the quality: "The 18th century composition of the metal was denoted by a variety of words, sometimes included in the angel or rose marks. This practice was most common in Germany, Austria, Hungary, France & Switzerland..." bla bla... "Feinzinn, Probzinn; Alte Probe, to indicate that the piece has been recast from an old alloy;
Blockzinn,
Blocktin; Krozinn, usually a ratio of 15:1 or 12:1..."
2. Couldn't find anything matching your raised flower inside, but "The Bodenrosette [was popular] in Switzerland, Austria, Hungary, Germany, Eastern France. The design is varied and sometimes includes the initials of the maker."
3. The placing of marks: "A tankard or mug marked below the handle just below the hinge [was] a favourite spot for marks in Switzerland, Germany, and Austria-Hungary." (Very rare on an English flagon) HOWEVER, sometimes ownership of the flagons was indicated through touchmarks of a coat-of-arms, crest or badge of the family, or armorials, punched or engraved. Perhaps this is what's on your handle below the hinge?
4. I looked at all the touchmarks in my book and could only find a few that that could be close. One is the angels, which indicated the quality with her scales and swords (representing justice), and a shield that (kinda) has a similar shape to the mark on your handle. It's the stamp of Johann Jacob Sprandel (Ulm, 18th century) but yours is too worn to really tell. I almost found another photo of one of his pieces from an auction in Germany, but the photo has been removed! Would've been great to compare! Grrrr. Anyway, here they are. Hopefully you can make out something! So many of them are close, but no cigar...