Hello and welcome to the forum,
I find the cabin filter gets pretty filthy pretty quickly but it still does it's job so I wouldn't read too much into the fact it was minging, doesn't mean it wasn't changed when it should have been.
You're doing well with your list of jobs if this is your first car, things get easier (and quicker) the more often you do them so don't worry about the time it takes you to do them the first time around, About a year ago I started doing repairs for the first time on my Jazz that I've had for 13 years, I was gutted when I ran out of jobs to do, had to start working on my mate's Vectra just to get my fix.
Keep up the good work and keep us updated.