Cleaning Dirty Paint

Did you leave the tape on until it dried? If so, that's your problem... You need to remove tape immediately after painting (well, you can wait a couple minutes) so it can settle down flat and not harden along the tape edge...

As for cleaning the paint, the only way I ever can fix that is to get in the with a 3 or 5 "0" brush and touch-up the edge..

First of all, you should check if it's allright to mix enamels and acrylics.

Anyway, you may wish to consider using a different type of masking material, such as parafilm/etc., or just cutting along the edge of the tape before you remove the mask. Either way, I think you'll have to repaint it. It might help to burnish, or press down, the edge of the mask real well.

Also, I don't think it's a good idea to sand a final coat.

It might be handy to know what your model is, and the specifics.

Can't think of anything but mask and paint the white again.  When you say edge do you mean ridge?  If you are getting a ridge you are putting down to much paint.  Maybe a touch thinner but definitely lighter coats.  Spray at an angle away from the mask, not into it.