So technically, I could just open all these things using her current name even if we've changed her name?
To make life simple, you should use the name she has in her foreign passport, whatever name that is whether changed or not changed.
And then change to new name on the items/bank accounts once she has got citizenship and put her new name?
Why are you concerning yourself with Citizenship now?? That is still a long way away. She can also only apply for citizenship in the name that appears in her foreign passport.
How long would it take to get a replacement brp?
Anywhere between a couple of weeks and 6 months.
Also if she has new passport with new name, that can't be used for extension can it? Because initial spouse visa was approved using different passport and the visa vignette will be on there instead of new one.?
Of course it can. Why on earth would she not be able to use her new passport??? You really are overthinking this completely.
What would be the easiest way to do all this and less hassle..?
The simplest would have been to change her passport BEFORE applying for a visa.