While I can see why there intuitive cause to abandon the "I am person #2, therefore there are probably not 100 people" reasoning, abandoning "There are 100 clones, therefore I'm probably not clone #1" seems to be simply abandoning probability theory altogether, and I'm certainly not willing to bite that bullet.

I'm not entirely sure what you're saying here. UDT suggests that subjective probabilities are meaningless (thus taking the third horn of the anthropic trilemma although it can be argued that selfish utility functions are still possible). "What is the probability I am clone #n" is not a meaningful question. "What is the (updated/posteriori) probability I am in a universe with property P" is not a meaningful question in general but has approximate meaning in contexts where anthropic considerations are irrelevant. "What is the a priori probability the universe has property P" is a question that might be meaningful but is probably also approximate since there is a freedom of redefining the prior and the utility function simultaneously (see this). The single fully meaningful type of question is "what is the expected utility I should assign to action A?" which is OK since it is the only question you have to answer in practice.

Actually, looking back through the conversation, I'm also confused as to how time discounting helps in the case that one is acting like a Boltzmann brain - someone who knows they are a B-brain would discount quickly anyway due to short lifespan, wouldn't extra time discounting make the situation worse?

Boltzmann brains exist very far in the future wrt "normal" brains, therefore their contribution to utility is very small. The discount depends on absolute time.

I wasn't being very precise with my wording - I meant that one would maximise the measure of whatever it is one values.

If "measure" here equals "probability wrt prior" (e.g. Solomonoff prior) then this is just another way to define a satisficing agent (utility equals either 0 or 1).

Well, I have only a layman's understanding of string theory, but if it were possible to 'escape' into a baby universe by creating a clone inside the universe, then the process can be repeated, leading to an uncountably infinite (!) tree of universes.

Good point. Surely we need to understand these baby universes better.