Can Prior Service get an 18x contract? Also, can prior service retake the asvab?

There's no rule to taking the ASVAB other than you have to wait six months between tests. However, USAREC has a policy that they won't retest someone who has a qualifying score. So, unless you're in the NG and want to use a DOD testing facility on a base to retake the ASVAB, you're stuck with the scores you have.

That actually works in your favor of preventing you from making a major mistake. Do not take an 18X contract. You will sign on for five years, most likely fail at some point, and get pushed over to the 82nd as an 11B1P. It happens so often that it's a running joke that the 18X is the best recruiting tool for the 82nd.

Take whatever job you would like to do if you don't make it through SF. Then volunteer for SFAS when you get to your first duty station. If you pass, great. You reenlist for five years for the course and get a bonus when you graduate. If you fail, you go back the the job you wanted and you didn't obligate yourself to a five year contract when you first enlisted.

A simple google search of “prior service rules for 18x” would provide you with all the info required

A new ASVAB test isn't necessary if we can verify your previous scores. However, if you wish to qualify for a new MOS, you have the option to retake the ASVAB. In my experience, I've assisted prior service members, not currently serving, going for OCS with asvab retesting. I'll review the regulations tomorrow to confirm there haven't been any changes.