Remedy for Cat Urine Stains and Odor on Hardwood Floors?

We need help! We just recently bought a house and the previous owner had over 30 cats that she let go wild around the house for probably over 15 years, we think. She is deceased and was a recluse, so getting information on her is difficult. But what we do know is there is the worst cat smell you could ever imagine. We had to rip up some of the hardwood floors in some rooms as well as the baseboards in those rooms. We were hoping to spare some of the rooms from having to rip up the floors.

We have two serious concerns. Into the corners of the stairs where the stairs turn was evidently a regular peeing spot so the urine has soaked down into the wood that makes the stairs, not just the tread but the wood that holds up the stairs. If we have to dismantle and replace the whole staircase it would be deadly expensive. The walls around this area are down to the studs so we can see where the source of the problem is. We have used enzyme sprays on the area to try to break up the urine enzymes to get rid of the smell, but it prevails. Then we started covering it in baking soda but that did not help either. Keep in mind this is a vertical problem we are trying to fix as it is along the wood that makes up the stairs going up the stairs.

So does anyone know a product we could use to get rid of the smell so we don't have to dismantle and replace the whole staircase? The second issue is that on the second floor (where it seems less cats were). We were hoping to save those floors, but last week we had some extremely humid days and floors that were dry and non stained on a cool dry day were all covered in wet, stained areas that seemed to seep up through the floors. Now granted these were very humid days, but still it was ridiculous the amount of stains and wet marks that were all over the floors. Do you think we just need to pull up all the floors? Please help!