In my view Rick's itineraries are not right for people who want to travel at a leisurely pace. I'd be a lot happier stretching one of his 14-day itineraries over three weeks than dropping a week in Spain into the middle of it. Travel between Portugal and Spain is a bit awkward. By the time you subtract the travel time, that extra week will be no more than six days. That's not a leisurely pace for Seville, Cordoba and Granada. A go-go-go pace that is barely manageable for two weeks would be considerably more of challenge to maintain for three weeks.
I seriously wouldn't do this. What time of year are you planning to travel? If in the summer, Andalucia is likely to be blisteringly hot.
I haven't seen a single comment on this forum by someone who has used BlaBlaCar. While I certainly do not read every thread, I do skim a very, very large percentage of them, so I don't think you're going to get many (any) reports based on personal experience here. You might have more luck on TripAdvisor. However, BlaBlaCar is just a platform for connecting with people (read: non-professional drivers) offering rides in their cars. I wouldn't count on a driver to leave at the indicated departure time, to have a car in great mechanical order, etc. Nor would I assume the car would be air conditioned.
There are no trains from Seville/Cordoba to Faro. Part of the challenge of combining Portugal and Spain on a single trip is that there are very few ground-transportation links. There are buses between Seville and Faro. I haven't traced the info back to the source websites (which you should do), but it appears there are three companies running buses along that route: ALSA, Rede Expressos and FlixBus. In general, western European inter-city buses are quite comfortable.