I hope I can help a little bit...First off, I can say~ been there, done that!
There is a really good book called Mothering Your Nursing Toddler, if you have time to look at it, but you sound pretty busy.
Just some random thoughts...
It might help to think of weaning as not being an all-or-nothing situation. Maybe if you compromised (just cut back til it was workable for you) you would still have the best of both worlds.
What if you wean him and he *still* needs constant mommy-time? At least now you have an instant soother (snack, drink, get-to-sleep, fix-a-bump, sit-down and rest, etc). Will weaning *really* make it easier on you?
For your back, maybe try just watching how you sit/lay. Bring him to you, not you to him.
On setting limits... Try distraction and getting him to wait a little bit first, say he needs to get a drink of water first, leave lots of good snacks out, plan other activities, say he can nurse while you sing the ABCs or count to 10 and then he can nurse again later...
It is rough, I know. On the nap thing, there are some days my dd (also about 18 mo) just will not nap without me and I need to get something done (or just can't sit anymore!) I just get up and forget the nap. If she *needed* one, she would have been in a deeper sleep, and wouldn't have missed me, right? So far, so good with that. She has never got overly fussy or anything.
You are the mom, and you know best. If you really want to wean, just remember gradually and with love, and your baby is so much better off for all the breastmilk he already got!
(p.s. it is late, so I hope this is all coherent!)