I asked all these questions and many more questions. I could never get a really great sound out of the mixes, nor out of single IR's and always had to combine at least 2 IR's and then use a lot of EQ or tonematch. I did get lot's of advice all over the internet and also from some of my pro friends, but the answers I got just didn't add up, and I was not satisfied and decided to find out for myself.
I think I was right in that the traditional way to mic a cab's "sweet spot" for live guitar applications is not the best way to capture a cab for FRFR. Even doing it the way they do it in studio sessions, the IR's are ok, and do capture a decent snapshot of the character and tone, but they always need to be mixed and never work singularly for me. In the end, I rented some decent mic's and a couple cabs, along with my own collection of mic's and cabs, and did a bunch of shoots with cab lab. I had a very good experience doing this, which helped a lot to figure out which IR's will mix together best and why I would actually want to mix them.
What started me down this line of thought was, I played for many many years just mic'ing a single speaker off a 4x12 and I could never get the same experience from an IR taken from the exact same position. My best mixes are from using Mic shoots that capture an entire room in conjunction with the Cabinet. Believe it or not, using a cheap $70 Behringer ECM8000 RTA condenser mic gave me the IR's that really have it, it's nearly the flattest mic you can buy for that price, and it eliminates any coloration, which then means it all comes down to placement.
Seriously I recommend to rent a couple different mic's, and get at least one RTA if you can and do some shoots yourself, using a cab that you know well. It is an eye opener.