There are some warnings about chiari and chiropractic adjustments, I've been seeing a chiro. For years, should I stop? ...

STOP. Manipulation of the neck is dangerous in any cirumstance and especially with Chiari malformation. Do not run this risk. Chiropractors should never be cracking necks as it is dangerous and many cases of neurologic damage and paralysis have resulted from this practice.
Not necessarily. Chiari malformation causes the lower brainstem and cerebellum to descend into the spinal canal. Unless there is too vigorous of a high velocity low amplitude correction then I don't believe chiropractic care will exacerbate the condition. I hope this helps. Take care.
Hard to say. Generally any drug, herb, treatment, or procedure that stands up to rigorous scientific scrutiny is included in modern medicine. The problem is that many alternative practices, Chiro etc do not stand up to scientific scrutiny and cannot be shown to work. They will take your money though and prob recommend repeat visits. Still, you will know quickly if it doesn't work. If no result -- stop going.
Depends on Severity. With a chiari malformation it may be best to look into low force, or instrument based adjustments. Obviously, keep your Doctor of Chiropractic informed, to discuss treatment options and bringing any MRIs or CTs would be in order.