oh you poor thing. I totally sympathise - I'm in the same boat myself although it sounds like your having it worse. DS2 is 10 weeks and wakes at stupidly short intervals, although I tend to get three hours out of him after he wakes around 10:15 (actually its not really is it, cos you have take away the time it takes to feed), then two, then one hour sessions. I do dread bedtime really. I also have DS1 who is nearly 3, but DH bless him looks after him in the night cos he's being a pain at the moment. He sleeps in a hammock until the wake about 4am at which point he wont settle back in there and I bring him in with me. I do find that I dont get much kip with him in bed with me though, I have to hold my breast for him to feed which isnt comfy and he wakes when I shift about.
Why not try giving mix feeding a go? It may well not work, but then if you dont give it a go, you wont know.
Personally, I mix feed, but he only gets bottles during the day. Thats because he is super-windy, and formula makes it worse, and also because he will sick up quite a bit of formula but tends to only posset a little bit of breastmilk. I dont express because I cant produce enough so its not an option.
I dont think mine would actually sleep longer if I gave him formula at night, so I dont bother, because I can function (just) on the rest I'm getting at the moment. I'm just praying he starts to sleep for longer periods soon. Good luck with it. x