I'm afraid I have only two words of German, so I must make my request for help with translation in English.

I'm working on 'The Young Cosima' by Henry Handel Richardson for the MobileRead library. It was first published in 1939 and the author died in 1946, so that it is in the Public Domain for MobileRead purposes. It is about the tangled relationships between Franz Liszt, Hans von Bülow, Cosima Wagner, and Richard Wagner. The book contains some German words or phrases which I can't make any sense of using Google Translate. I want to put the translations as end notes or annotations.

The first word is Commerzienräte as in 'Oh, I could have spat at them: this crowd of paunchy Commerzienräte'. Hans von Bülow is bitterly complaining of the poor applause given at one of his concerts. What is he calling the people in the audience?

Also, should this word start with a K, in today's German? Would it have started with a K in 1939?

The second word is Mendelsvaterenkelism, as in 'what our young friend Hans so wittily calls Mendelsvaterenkelism'. Both von Bülow and Wagner were against Mendelssohn and his kind of music, and so far as I can tell it is an invented word. Does it have any meaning in English?

The von Bülow family motto is quoted as 'Alle Bülow'ehrlich'. How should I put this in English?

Hans von von Bülow is bitterly criticised by his mother for 'hacking around the country as fahrender Künstler'. Google translates this as 'driving artist'. Is there a better translation?
"Commerzienrat" is, or was, an honorary title conveyed to business men only after substantial "donations for the common good". So these guys would be rich businessmen who had polished their public image through what one might call today "philanthropic" donations. Modern German writes the word with an inital "K", but I have no idea about the linguistic sensibilities of 1939 Germany. I'm also not entirely sure why he's calling them that, but I imagine it's his impression that they are either too business-oriented to appreciate fine art or simply too full of themselves, or a combination thereof.

"Mendelsvaterenkelism" is a word play. "Mendelssohn" literally means "Mendel's son". "Mendelsvaterenkel" would be "Mendel's father's grandchild", so either Mendel's son or his (half-)sibling or cousins. So the word would be used to describe "Mendelssohnism" and related ideas - whatever that might mean to those two - without actually fully naming the person who is the target of their ire.

A straightforward translation of "Alle Bülow'ehrlich" would be "All Bülows honorable".

I would translate "fahrender Künstler" as "itinerant artist".
It starts with K in today's German and I found it with a K in the 1910 edition of the Muret-Sanders German-English dictionary.

I.e., it's safe to assume that it'd have been written with a K in 1939. Of course by then the honorary title Kommerzienrat had fallen out of use.

BTW, the Muret-Sanders dictionary defines Kommerzienrat as being roughly equivalent to Councillor of Commerce. IMHO, a better definition might be Captain of Industry.
I agree with Gudy on the first two points but would opt for the simple "travelling artist"; as far as the "fahrender Künster" is concerned. Itinerant is technically correct of course, but probably lacks the negative connotation as in "he is -just- like a travelling artist". The mother doesn't see him meet the expected family-standard.

As to "Commerzienrat": The "C" would habe been preferred over the simple "K" as being definitely more, well, posh. I think there was some adjustment in writing loanwords which had the latin "C" in it rather with a "Z" or a "K" (where the sound was appropriate) at around 1900.

(As an aside: One of the bigger financial institutes in Germany still bears the name "Commerzbank" and has its roots in the "Commerz- und Disconto-Bank" (note both "C"s) which was formed February 1870 in Hamburg, so one still does see the "C" sometimes.)
IMHO, Gudy's suggestion "itinerant artist" is perfect, because it's a commonly used collocation.
@Doitsu Yes, ok. Just a suggestion on "fahrender Künstler" for AlexBell. Up to him to decide.
Many thanks for all your translation suggestions. I've printed them out and will make changes to the annotations when I've collated them.

For Commerzienräte I think I'll leave the C; it fits with the context well.
Could I have more help, please?

In a letter from Wagner to von Bülow he writes 'I want you here to Niebel and to Tristel for me...'

I can't find anything in Google translate, and a Google search brings up both words as surnames - in German so I can't read the examples. Any suggestions on how I can put this in English?
My best guess is that the author turned Der Ring des Nibelungen and Tristan and Isolde by Wagner into verbs.

I.e., he most likely wants von Bülow to play some Wagner pieces for him.
What a marvellous idea! I'd never have though of that.