I've looked at your links, and at other places, too.
While 'staff' is commonly used as non-count, especially in AmE, it is comparatively rare in combination with 'much.' or 'little'. Even in one of your links, in which the title of the article was '
How much staff should a doctor have?', that was the only use of 'much staff'. In the article itself, the writer used 'How many staffers'.
From my admittedly superficial reading, it appears that people prefer the plural form when they are fairly clearly thinking of
a number of members of staff. So, in #1 below, many speakers of BrE might use 'are', but most would find 'is' acceptable.
1.
The staff [seen as a whole body] is behind you on this.
However, as soon as we introduce the idea of plural people (as implied by the underlined words in #2], 'is' sounds less acceptable:
2.
All/Most/Some/ of the staff are behind you on this.
In the original sentence,
"There is far too much staff here in this room", there is a clear idea of a lot of people. Richard's "
one would never say too much staff, it's just wrong" is, on reflection, too harsh. Rewrite it as , "almost no speaker of BrE would ever say
too much staff,
it just sounds wrong", and I agree. Wholeheartedly.