I can think of several solutions:
1)if the nasty downstairs neighbor doesn't want to switch is there another downstairs neighbor that will? My husband always wanted the top floor apts. so he didn't hear ppl walking above him. Maybe there is someone else downstairs that feels the same way.
2) even if the landlord won't carpet your hardwood, you can purchase large area rugs - some even quite thick - at Target, WalMart, Home Depot, Lowes, etc. for quite cheap. Just lie those over the hardwood for some sound insulation.
3) talk with the downstairs neighbor. Find out when she is home most often and try to work out a schedule when it's permissable for the romping and when she needs them to be more settled. That way, Nikki will get into a routine of when it is playtime and when it isn't and you won't have to crate her for too many hours.
4) take Nikki for long walks when you get home or to a dog park if you are fortunate enough to have one close so she can burn off her energy and be ready for sleepies when you bring her home. Or, if you can afford it, hire someone to come walk her before you get home to help burn off that energy. If there are no professional dog walkers in the area, hire a responsible teenager from your apt. building, church or a high school/college.
5) if the landlord says you absolutely cannot keep her there, ask him if you can get out of your lease with no penalties because he is renigging on his agreement to let you have the pet so you can find a more Nikki-friendly living arrangement. It sounds like he is pretty reasonable and trying to find a solution everyone can live with instead of just giving you - or Nikki - the boot.
6) bring her to my house. I'll keep her!!
Jeanne