Hi there Jean and thanks for the post.
You are doing a great job and it is great to see such committment. I am also very pleased you have a set pattern as changes can often be the base problem.
Now even though you are in an apartment think about using a a astroturf toilet area (there are even special ones made for apartment dogs) or you can stick with the paper.
Now when you are home i would take him to the paper or turf every hour and wait. If he shows any interest praise him and make sure he knows he is doing well. If though he doesn't wait a bit and then come back to it.
The toileting when he plays is a young dog overexcitement and poor bladder control issue so don't worry about that.
For the next few weeks i think it may be a good idea to confine both dogs to a smallish room with food, toys and a toilet area. Confining will encourage him to use it as he has less distractions and as you said may learn of the older girl.
If you catch him messing don't growl as for him he sounds a little nervy instead take the mess to his toilet spot and leave it there, smell is great to help train them.
As he learns to use the pad then he can have more access to the house but only when he has learned a little. You may want to use a product like DAP to calm his nerves a bit in his toileting area so he may feel more comfortable about doing it.
Let me know how things are going and if this helps if not i will endeavour to help more
Keep up the good work
Todd