From my voice teacher, ENT and gastroenterologist, I learned:
Acid reflux can definitely irritate the folds and make them feel phlegmy.
Alcohol (regardless of whether beer, spirits, any color wine) not only is an irritant but is drying....AND relaxes the espophageal sphincter, which allows acid to back up into the throat when sleeping, causing or worsening acid reflux. Citrus iby itself is acidic, and if you have GERD it will make your voice worse, not better. Caffeine does not tighten the throat--it dries it out, and can irritate it just like alcohol--both are diuretics. You want to be well hydrated, and it can take 24 hrs for water you drink to reach your folds.
Dairy is a crapshoot. Some people get incredible phlegm from the slightest amount. Others are totally unaffected--my voice teacher can knock back an ice-cold milkshake and sound terrific. Listen to your own body,
If you are asthmatic, ask your pulmo or allergist about Singulair--since I started taking it I haven't had to use an inhaler for months at a stretch. Yes, inhalers, antihistamines, and decongestants are all very drying, but your total health (and ability to breathe) trumps your voice. For everything you do that might be drying, try harder to hydrate and lubricate--water, slippery elm, hard sucking candies ("boiled sweets") w/o menthol (mint is an irritant, especially to the esophagus and can make GERD worse).

Oh, and nobody produces sound from the diaphragm itself--that's what moves the air. Your folds produce the sound and your resonators (in your head and mouth) shape the tone. Make sure you aren't pushing too much air--that can actually blow the folds apart and inhibit vibration!