Well, have a look at Medscape which explains that the sensitivities for Anti-RNP is 95-100% for MCTD and 20-30% for scleroderma. As already said I am negative for ANA & SCL-70 but have diffuse scleroderma so I know that's possible and it's also possible to have any connective tissue disease without the corresponding antibodies. I guess someone could therefore have scleroderma despite negative ANA & SCL-70 but positive Anti-RNP but of course I have no idea whether you do, just that it must be possible.
Yes antibodies can change over time and yes you can have more than one antibody.
I am not sure if I mentioned this recently but from a treatment perspective it matters not what connective tissue disease you have because there is no treatment specific for scleroderma rather they are all treated the same, immunosuppressants, DMARD's, symptom lead treatment. Have a read of our difficult diagnosis page, especially the segment "They feed all dogs dog food."
I hope this helps and take care.