Yes, you can make sticky toffee a day in advance.
Restaurants which do a big prime rib business usually make the au jus the day before (at least) also. Unless you're holding the roasts, appropriately wrapped and in appropriate coolers to rest for some time (which I recommend if you can get your mitts on enough coolers), you're going to be strained to start the au jus with the drippings and get it properly reduced while not over-resting the meat.
"Appropriate coolers" is not a big deal. Even the cheap foam ones will do. And everyone usually has at least one or two large coolers in the garage, so no problem borrowing them. I like to wrap in heavy-duty cling film. If you can't get the heavy duty stuff, aluminum foil is almost as good. Don't use regular Saran-wrap (or a super-market generic) because it tends to melt at low temps.
Stretch-Tite Premium is good stuff; otherwise just look for something "professional."
If you're resting your roasts tightly wrapped in well insulated coolers, you can hold for a couple of hours without any decrease in quality.
In my opinion, unless you're rib roasts are very lean, 110F is a bit too rare for hot service, even allowing for the carry-over temperature rise. 110F is fine for inexpensive meat which isn't well marbled, and/or meat which is going to be served cold and sliced very thin; but allowing for carry-over temp increase, you're looking at 120F (max) which is not only very rare but also barely warm. Worse, 117F -- which is borderline raw -- is more likely than 120F. You wouldn't think 5F would make that much difference, but I strongly recommend a 115F minimum internal pull temp. After the rest/hold, that should give you meat which just splits the difference between rare and medium/rare.
The word gravy is getting tossed around. I'm not sure what's meant by it. I tend to think of gravies as having more body than an au jus (which is not thickened and only slightly reduced). You don't want a thick gravy anywhere near your prime rib -- not even for the potatoes.
And getting back to preparing the au jus, an acceptable alternative to making it au minute and using the pan drippings, is simply diluting a good commercial demi-glace such as Demi-Glace Gold.
When I catered (eons ago), I did quite a few prime-rib dinners at similar sizes to yours and made both the au-jus and Yorkshire pudds while holding the roasts. It's not the only way of doing things, but it works. Even so, it might be a good idea for a non-professional to go with the Demi-Glace Gold. No matter how much you get done in advance, cooking this sort of meal for 70 people is quite an undertaking. The less you have to shuffle the roasting pans around, the better off your service will run.