IMO, Scarf Infernape is the best one right now as it doesn't rely on breaking holes in teams (hard to do with rain being so common) but rather patches up common weaknesses that Sun teams have. Don't forget that Infernape has that same base 108 Speed Terrakion has to revenge Modest Venusaur in the Sun, only Infernape has Fire STAB to destroy it for sure with no prior damage. As long as Ninetales is higher up in the usage stats, Infernape will be close behind due to the utility the Scarf set brings to a Sun team.
I agree that Scarf is the best set, but I surely didn't forget that Terrakion shared the same speed tier that's why I actively compared Infernape to Keldeo and Terrakion. Thing is, Infernape's scarf set can't be
that close, it would have to be used on almost all sun teams to be that high! All the things that you listed that give troubles to sun teams are also handled by Terrakion (and Terrakion can hit Extremespeed Dnite). Infernape's only niche is his Fire-STAB, but TBH hitting opposing sun sweepers isn't the biggest threat for a sun team of your own. In general, Scarf Terrakion can hit a lot more targets because he has STAB Stone Edge to back up killing Tornadus-T, Dragons, and Volcarona (as opposed to choosing between the targets with HP Ice and Stone Edge like Infernape does).
The only niche Infernape has is Scarfed U-turn, which isn't a lot. Subpar mixed attacker due to his STAB's being resisted by many Pokes (as explained in my post above) and has to compete with Terrakion on his Scarf, Banded, and SD sets which (IMO) Terrakion just does better. Just came to mind if you are using SR on Infernape he has to compete with Terrakion too now because of the tutors :o
Basically not a lot is going on for Infernape these days. Just Scarf and the super-rare Nasty Plot Infernape. But even with that little going on for it, it still manages to remain close to the top 20 in OU o.O I mean look at all the Pokes under it. Many of which are more relevant to the OU metagame than Infernape is from my experience playing. The big ones being Keldeo, Celebi, Landorus-T, Jellicent, and Haxorus! IMO I think it is time for Infernape to go from OU. I could give the same rant about Vaporeon but that's for another day :P
Then again, this is coming from a guy who thinks Metagross is solidly OU and thinks people bash it too much... (which is also a rant for another day)