After a masters I think the only other option is another masters. Whats the point in taking an undergrad biology course when you just did a masters program? If they are truly serious they need close to a 4.0 to make up for previous academic under performance. They need to show dental schools they can succeed at a higher level.
In my opinion, unless you are in a joint PhD/MD(or DDS) program, a PhD should NOT be a stepping stone.
PhDs require a lot of dedication and passion in a certain field. If you aren't in the program for the love of the field, it will be very hard to complete it. Additionally, it's around a 4 year commitment...just to turn around and do another 4 years in dental school.