So, my question boils down to my title line: which is safer - a less than perfect RF install or very secure FFing install?
I'd take it to a tech to get help with a solid RF'ing install. At 13 months, I really think that would be best - screaming and all.
Where were you checking for movement for the rfing install? You only check where the seatbelt goes through the seat, and as long as there is 1" or less of movement the install is fine.
At 13 months I wouldn't even consider ffing. I woudl try installing the seat more upright (30*) and put up a mirror to see if that helps.
At 13 months I would find a way to get a good RF install. I don't consider FF an option before 2 or so.
I have that seat, what's the trouble with it RF? Maybe someone can help you get it tighter.
BTW, DD screamed in the car around that age too. It's really tough! But they really do outgrow it. Now she's happy as a clam at 2.5 (still RF, in a Complete Air).
What kind of seat are you using? Maybe folks here can offer you some tips on getting a better RF install.
What year? Are you locking the seatbelt?
You can twist the female end up to 3 full rotations to shorten it. Even one full twist can make a world of difference.
You don't need a locking clip, unless you're trying to correct tipping. Tipping isn't a safety issue but it can be annoying. If you are going to use a LC, my advice would be to tighten the belt, unbuckle the belt while holding it so you know the length you need, and then shortening it just a tad, putting on the LC and buckling. Those Subarus can have really long buckle stalks so twisting might make a big difference.