Caffeine is a stimulant, so it should help counteract the drowsy effect
of OTC antihistamines.  Other than that, I can't think of any problem.

You name it, somebody out there is allergic to it.  But if you had a
true allergy, you would be a lot sicker than you are--hives, a rash, etc.

The next time you get that creamy taste, look at your tongue and inside
of your mouth and back of your throat with a mirror and flashlight.  Do
you see an actual coating or whitish spots, or does it just feel that way?

There are at least two possible ways you can get an actual coating in
your mouth:  A thrush (yeast) infection of the mouth; or a bacterial
biofilm caused by infected post nasal drip sticking to the back of your
tongue.  My dentist has seen both scenarios.

(I'm assuming you use a tongue scraper so it's not food particles
sticking to your tongue.)