German (and Austrian) Gems

The Baroque stands out stylistically and technically among these labels, because it was based upon such instruments as the harpsichord and double-reeded winds (the oboe and bassoon) rather than the piano and single reed (clarinet); and because Baroque music centred on the 'knitting' of contrapuntal lines rather than a single melody with stylised rhythmical accompaniment. The other labels quoted in this question are all more or less Classical and even heading towards the Romantic. All of them are musical movements in which German-speaking composers and performers played a significant role

Sadly, Schumann's mental health declined (possibly aggravated by medical treatments, of the time, for other conditions) and he died a broken man in an asylum. But his cheerful and more pensive music each live on deservedly

Most of the others were still writing 'tunes' (pieces incorporating the kind of melodic line that one might hum or whistle) while Schoenberg was experimenting with atonal music in which each of the 12 pitches held equal importance. From the empirical, mathematical and even socio-political point of view this is intriguing and democratic, but it is an acquired taste and rarely soothing on the ear. The Bluffer's Guide to Music (first published towards the end of the 20th century) rather neatly defined Atonal (i.e. home-key-less) Music along the lines of 'that for which someone, in the great hereafter, will have to atone'!
Orff, meanwhile (he of Carmina Burana fame) was an educator and percussionist, Weill was creating minimalist opera in the Weimar period alongside the playwright Berthold Brecht, and Hindemith was a pioneer of Gebrauchsmusik ~ 'music to be used' on various occasions, rather than written &/or performed merely for its own sake. This was conceived to appeal to all musicians including amateurs, and such alarming newfangled techniques as atonalism would almost certainly have killed the idea stone dead in terms of capturing the public imagination and sympathy

Papa Haydn probably had a hand in more musical developments than any other classical composer, but seems to have slipped up in the piano concerto department

The Symphony (as a form) did not come into being until after Bach's time; a number of his sons tried their hand at it

These were all trailblazers, in their respective times and styles, of opera in German-speaking Europe

The introit often sung-along-to under the breath as 'Here comes the Bride' is in fact the Bridal Chorus from Wagner's Lohengrin, and the recessional often requested with it (as a 'safe' Victorian-era pair) is the Wedding March as in the question, originally depicting the nuptials of the Queen of the Fairies with a man ('Bottom') who has donkey's ears.
Answer 4 is a non-starter as Wagner only wrote mostly on a much broader scale, and did not trifle with string quartets and the like; in any case, the heyday of the vinyl album was generally within 50 years (and hence, living memory) of the Nazi era, so no self-respecting label would have been so culturally insensitive as to couple the works of these composers onto the same disc ~ except perhaps as a sampler for upcoming weddings!

Strauss famously celebrated the 'Blue Danube' and Wagner's Ring Cycle makes plentiful reference to the Rhine (also famous for its Lorelei rock, and numerous settings of the ballad about it written by Heinrich Heine)

The tone of a Bluethner grand is distinctive once you know what you are listening for

For any footloose musical pilgrim in that part of Europe, Leipzig is a definite 'must'!