Will try to address several of the issues raised:
Physicians no longer use/recommend oral benadryl much: too short lasting, needlessly sedating.

Much longer lasting, and much more potent (est. 10X more) antihistamines are cetirizine (Zyrtec), and levocetirizine (Xyzal).
Both are also MUCH less sedating than Bendadryl, especially levocetirizine.

Both are more expensive than generic Benadryl, but both are available as liquids, both are OTC.

Cetirizine and levocetirizine work well on an occasional basis.

Aspirin becomes anti-inflammatory if you take 3-4 tabs every 4 hr or so for at least 6 doses. It may be the most hazardous to the stomach of the options discussed, and tinnitus, ringing of the ears is an issue at this big a dose. Aspirin also is one of the quickest medicines to break down in storage: one of several where the manufacturers "expiration date" has some relation to reality.

There is little data as to whether the expiration data on the two above liquid meds is real or not: an advantage of the tablet versions is that, except for aspirin, they seem to last for decades.

Yes, Mucinex (guaifenesin) is available as a liquid.

Naproxen tablets, and others, can be put into a plastic bag, like a ziplock, crushed into powder with a hammer, then dissolved into the beverage of choice.