Thank you!Firstly, "it" is very rarely stressed, except in such idioms as "She thinks she's it" (= she's snobbish) and "She's got it" (= she has sex-appeal). If you want to emphasise the thing that "it" refers to, you'll often choose to say "that" instead. Or in the case of sex-appeal, those! )
Of the two versions "How much is it?" and "How much is it?", the former is much more common.
I slightly disagree; the British vowels are always clearly distinguished from each other whether accented or not:Normally 'can' is unaccented, and has a neutral vowel: [kən]. The negative 'can't' has a full vowel. So this is usually the difference between the two in both BrE and AmE. However, I have the same problem as you: an AmE speaker might be stressing the word 'can' or 'can't', in which case they are practically the same before the [d] of 'do'.