Quoting Bill, "If I can't use full keyboard mode with SA voices, what would be the next best fingering mode to switch to"

Hi Bill,
According to the information button it's only certain SA voices that react in that (unacceptable to you) way.
Select SA Saxophone & press Information.
Note the words "monophonic or polyphonic depending on how you play".
Suggest you stop using that sax and substitute BigBandSax which doesn't have that restriction and moreover sounds exactly the same as the original one.
The SA Silver trumpet plays similarly to the SA Sax, so avoid that one but if you want the "same" sound switch to the Sweet Silver Trumpet on Page 2. (Makes you wonder what the special merit of some of these SA voices are considering there are substitutes which can sound at least as good).

So my suggestion is to look at the info. and substitute any troublesome voices - there are plenty to choose from.

John