I may have missed this part of the thread, apologies if so, but if the smell is just around the door, it might be neighbourhood males spraying up the outside of the doors, which is leaching through to the inside.
Suggest use the RX66 ( I can recommend) on the outside of the doors/step area then wash down the path around the door with a standard path cleaner.
Is it long since your kitten was spayed? She might be the cause of the scenting if not the source. Females exude their feramones for weeks after neutering so expect this to get much better once this period is over.
Once you know this is the case you can act to temporarily barricade the marking by taping some cardboard flaps to the outside bottom of the doors, these can help deflect the wee from seeping down the gaps and into your home. Spray with a little fabric waterproofer to make them last a little longer.
Oooh and
to Purrs!