Where do I go from here?

Melon had little scabs about her face & neck.  She was always scratching.

I applied Frontline to all the cats.  She scratched as much as ever.

Then her toes swelled up, every toe on each front foot.  They were bright
pink and so swollen you couldn't express the claw.

I went to the vet, who gave her a depo shot.

She was of course great for about a month.  Then other areas swelled up,
this time the toes not so much.

Off to the vet for another shot & scrapings, which found "bacteria".

Frankly, I expected bacteria to be found in a swollen area, and even though
I'm no vet, I expect the bacteria are opportunistic and not causal.

She again is swollen--this time the chin (puffy, not too red) still mildly
scabby about head & ears, but one really raw spot on her back foot.  The
toes are starting to react.

The last vet suggested lupus.  Really?  Is there any other possible reason
for this?

I switched the dry food.  I haven't switched the kitty litter.  We use
Swheat Scoop.  Is it possible this cat has a serious wheat allergy?  Is this
common in cats?  Can it be tested for without stopping the use of this
litter, which is very very good for my 11 cat brood?

Are there other things that might be going on?  For instance, Elmer has had
eoenciphilitic plaques for years.  Melon doesn't have this, I don't think.

Also, apparently the best treatment for autoimmune diseases or periodic
allergies of this type is steroidal.  Instead of getting shots, wouldn't it
be better to give a decreasing steroid regimen (as I must for my poison ivy)
when she flares up?

I'd like to suggest to my vet that we just treat the symptoms this way.  I
currently am shelling out the bucks for my epileptic dog who had acute
pancreatitis this summer, and my cat just diagnosed & being treated for
breast cancer.  There's not a lot left it the wallet!  But I don't want to
be a pushy patient and pay the price by hurting my cat.

And is there in fact any point to testing when this is likely to be the
treatment of choice?

She shows no other symptoms behaviorally or physically.  Of course, her
behavior changes a bit when on the steroid!  But otherwise she seems just
fine.

Thanks for any clues of how to approach this.