James, you'll get a different answer from everyone about products. It's a very personal thing!
Don't get too wrapped up in trying to be a scientist, you'll spend longer burning the carpet than cleaning it :

i can remember several topics in recent months like "what's your top 3 pre sprays", have a scan through those for names/brands. Don't be afraid of the ultimate master it's very good stuff.
If it's a bit dirty, hit it with something strong! My favourite approach with a porty used to be m-power or heavy duty microsplitter (sometimes both, apply the HD over the bad bits after the MP has dwelled a while), a THOROUGH SCRUB and then a good old rinse out with UM. I used to use the UM below the standard dilution rate for that purpose, because you've already done half the work with the pre-spray. (tip: carry two 'clean' buckets, fill with hot water and fully dissolve the UM in these before adding to machine, i use a big metal spoon to stir with, normally takes 5 mins until the white stuff all dissolves and it goes CLEAR. this helps stop it screwing your pump too quickly)
As long as you flush thoroughly, the UM (allegedly) self-neutralises so you shouldn't have to worry about pH if there's wool present.
Sounds like you've done things the proper way and got a good grounding in how not to screw it up... get stuck in, you'd have to really ignore the "rules" to ruin a carpet, just plug away and find what works best for you
