Yes. Most of the time women don't know exactly which hpv type they are infected with. In addition, women often clear hpv infection all by themselves! For these reasons, you should be vaccinated regardless of hpv status.
Yes. Even though you may have one strain of hpv, receiving the vaccine will protect you from the other strains that can cause cervical cancer or anal warts. At least 50% of women become infected with at least one strain of hpv within one year of becoming sexually active. Doesn't mean that you are infected with the strains that Gardasil protect against.
Hard to say. At a listed age of 48, you are beyond the approved age (9-26) where insurers would cover it. You could benefit from the protection provided by the newer 9 strain Gardasil that would likely include strains you have not been exposed to. The vaccine doesn't treat or help with existing HPV infections. You might be able to find a provider to give it at your own expense, but it is costly.