Welcome to FF!
Since you have been adding Ammo Lock, I believe it can affect the readings of ammonia on the test kit (I could be confusing it with another product though
). I would stop adding it, so you can get a feel for how much ammonia is really in the tank. Go ahead and put the filter insert in. I also don't think it would hurt anything. Will it help? I'm not sure, but it certainly won't hurt.
Once you stop adding the Ammo Lock, check your ammonia daily and if you need to, do small daily water changes of maybe 10-15%. Your cycle should be over soon I think. It seems like you've had the ammonia spike and the nitrite spike (even if it was low). The pleco could have thrown the cycle off a little, but most likely not. I really think the Ammo Lock is giving you a funny reading.
I'd stop it and see what the level is then.
Definitely add rocks if you have a creek nearby like Sue mentioned. That will give you some good bacteria and help move your cycle along.