yep, i've only ever bothered using the front locking tab, using both seems overkill to me, the inlet cam is turned but some gears that mesh with the exhaust one, which the belt pulley is connected to, so the inlet cam cannot move if the exhaust cam is locked.
The locking tools are a super tight fit, first time i used mine i was convinced it didnt fit the slot in the cam but they do, you must be off ever so slightly for it to not go in. Clearly you dont want to use too much force but try really gently tapping it in with a very small hammer and gently giggle the cam pulley with a socket on if you are struggling. If it really wont go in then you could take a slither off the locker with a file or grinder but you shouldnt have to.... what you describe sounds pretty normal.
What i meant by them often being slightly off is that once you get the cam locked I have found that the pistons aren't bang on level, very close but not bang on. So once the cam is locked and you take the belt off that you may need to turn the crank a tiny amount to get the height indicator things from the kit exactly level. This is the part that the kit is really good for, its very hard to get it exactly level with just 2 long bits of something the same size - with the kit its pretty easy.
so lock the cam, remove belt, double check the pistons are exactly level and adjust as necessary, then fit belt, tension, re-tighten the cam pulley, turn engine over few times to check its all still timed, then put car back together.