You seem very certain that you want to stop breastfeeding him. Just take some time to make sure of that, since the easiest option will be go back to cosleeping and feeding to sleep. It's not actually a problem to tantrum feed a toddler and baby. There will be lots of online support to do this (try milk meg as a starting point). You could create a family bed for all if of the children.
Im not say in this is the 'right' answer, it's just that it might help you to know that you can make long term breastfeeding work, if you want to.
If you really don't want to breastfeed anymore, the only realistic way to do that is going to be to pick a date and bite the bullet. You need your DH to be available for this, so time it with when he has time off work. If not your DH, anyone who doesn't smell of breast milk. Could your mum move in on for a week or two to help? Any other friend or relative? How about employing a night nanny just for a few weeks?
Someone else will need to do all the nights. I eouldnt offer water as an alternate, that will just prolonge the agony and I very much doubt she's actually thirsty.
Instead I'd consistantly use a comforter. A special toy for example. Make a back-story to make it extra special ("this is mummies teddy when I was a little girl. Teddy needs lots of cuddles at night time. Will you be the one to look after teddy for me? Will you cuddle him tight?" etc).
That way, she has a way to independantly get the comfort, rather than needing to call you for more water etc.
In your situation, I'd look into hiring a night Nanny to support you for either 2 weeks or a month. I bet the investment will be worth it. Completely hand toddler over at bedtime. Sleep away from home if at all possible, so you don't have to deal with any pull to intervene. That will then leave uou better rested to cope with the daytime weaning.
If a night nanny isn't feasible, your DH needs to step up and do this.