What is a "safe" gift to buy them if one doesn't know whether there are any conflicts with:
alcoholism (well, at least two of the profs would be OK, I believe, with alcohol)
religious/philosophical/cultural restrictions (one is Hindu or Muslim, one might be Buddhist, and at least one might be Catholic-- Roman and Eastern Orthodoxy)
fruit, nut, cocoa, etc. allergies
and the like?
It's always so hard to get thank-you gifts for professors. On the one hand, some people question the appropriateness of it. I personally don't think it's an issue at all. But then, I'm a graduate student, and my professors have helped me quite a bit. I personally think that considering how much they have helped me, a gift is the least I could do to express my gratitude.
One thing I have given to my professors, friends, and family are custom-designed cookies. I personally make the cookies from scratch, hand-cut each one (I first draw a "mold" out of thick index paper if necessary), bake, and then hand-frost each cookie. This option allows me to personalize cookies for each person. For instance, I have standard cookies that I give to men (cookies shaped like dress shirts), and standard cookies that I give to women (cookies shaped like high-heeled shoes).
The cookies for each of my professors reflect their academic interests and/or hobbies. For instance, I had a couple professors who are experts in religion, so they each got at least one cookie shaped like a church; one got a cookie shaped like the Holy Bible. A political science professor received cookies shaped like the Democrat donkey and the Republican elephant.
Another professor I know is a musician, so he received cookies shaped like an electric guitar and an acoustic guitar.
Two of my professors received cookies designed to resemble the books they published.
All in all, the professors thought the cookies were hilarious. One professor said he never thought he would be eating his own book.
I did find out, though, that one professor is a staunch health nut. I learned this from a fellow grad student who is quite close to him. Well, it turns out this professor is pretty much vegan, and doesn't really eat sweets or products with high-flour content. Nor do members of his family, including children. So, you see, food gifts definitely don't always work, but this health-nut professor still found the cookies--particularly the one shaped like his book--amusing. I don't know what happened to the rest of his cookies (I know one grad student ate one), but it's the thought that counts, I suppose.
You could get them a gift certificate to somewhere like Harry and Davids. Or, a gift certificate to a nice restaurant.
Or, a 3 month subscription to a veggie or fruit club depending upon how much you want to spend... though that reminds me of that episode of Everybody Loves Raymond where his parents are pissed at getting so much fruit...
Or, how about like a collection of cookie cutters or something to that effect, or nice napkins or pizza stone or some sort of "make-it-yourself" kit?
Well, if you're looking at all those limitations, that really doesn't give you that much to work with food-wise. I'd suggest something like smoked salmon - but what if they're vegan/don't like fish?
Can't tell if you want one blanket gift that you can give to each professor, or if you're going to give different things to each prof. I'd guess that with all the restrictions, you'd do the latter.
You could give gourmet teas (such as something from Le Palais de Thés, or of that quality) or coffees, if you know they're of that bent. I personally like to give chocolates from my favourite chocolate stores in LA, or if I'm unsure on the chocolate aspect, pâtes de fruit [http://tinyurl.com/ym5s8o]. Otherwise, you're probably safer going with non-food gift items.
There are a few limitations in your post and I understand that(!), but getting by them, what about gourmet coffee, tea, nice bottle of olive oil or special vinegar. Go to a gourmet market and just look around - there is something that is bound to jump out at you. You are kind to think of them.