Looks pretty straight might be just enough to give you issues. Have the hanger checked with a tool. It might just need a bump in the right direction.

By tension, I guess you mean adjusting a barrel adjuster somewhere that is really changing the cable length, not tension. So if adjusting that will mean everything else is messed up, then I'm not sure what to look for except for maybe some frayed cable in the shifters or somewhere in the housings.

You can shift past the 21 tooth and all the other combinations are correct. Or do you not use some of the combos because you've never been able to shift to them? I ask this because I've found in actual face to face conversations that people will tell me that their bikes shift perfectly and in the same breath tell me that they can't shift to certain gear combos. It always leaves me flabbergasted.