Ok here goes an attempt to answer your questions, which should mostly be covered in the owners manual.
I will try to take them quickly in order, bubbles are air drawn in by the venturi tube through the ozonator, bubbles do not necessarily mean your ozonator is working, just that air is being drawn through it. If your Ozonator is original chances are it is dead as older ones had about a 4-5 year useful life (some models now made are better)
You can use either type, there are also a number of aftermarket brand filters that will fit
As to sanitation system, I would pick something with chlorine or bromine, for a simple method I would suggest googling the dichlor then bleach method for hot tubs
You mentioned that you have cleaned the tub, how, did you use a flush product like Ahh-Some?
You ask about UT1, -UT4, for liability reasons tubs in the US usually only allow temperature settings up to 104 degrees F, UT-1 -UT-4 are over suggested temperature settings where UT-4 is about 106.5 degrees.
