I have used the hormone chip implant on my molly mules with great success. Since  the mollies are sterile anyway, I don't have to worry about the quality and timing of their cycle. If the chip is put in too late in the season, it can really mess with a quality estrus early in the breeding season, even when the mare is kept under the proper lighting. On my mares, I prefer Regumate so I can regulate them better.

yes, you can stop the regumate in between shows but you need to really rely on your repro vet for the guidelines. You can stop the Regumate anytime. You just need to try to keep some rhythm to her cycle especially going into the fall if you may want to breed early spring.

The marble in the uterus isn't a good idea since the idea is for it to cause irritation to the uterine wall to prevent normal Estrus. Plus most times, the marble needs to be extracted by procedure.

The implant is the best, IMO, since you pop it in high in the neck near the crest usually but have heard of some vets placing it in the perineum area. Downside, it can migrate since it is the same approx. size as the ID chips. Each animal is different. My older molly mule takes a chip usually in Apr than again in late July. If we didn't put the 2nd chip in, she would have to be on Regumate several weeks prior to and the whole time she was at QH Congress stalled between the stallions. She is great, BUT she is a ho when in heat.... screams, drools, gets dead sided, gets super attached to other equine, but gets evil and down right intimidating to human strangers.  The cost of the chip (about $50) is minimal compared to the cost and the problems of handling Regumate daily. I have known people on the circuit with mares that were chipped and still needed Regumate at the shows. So you need to be aware that each mare is different and it will take a little while tweak.

You should discuss your options with a REPRO vet prior to making your chioce. If you don't have a good Repro vet available to you, then call one of the vet schools. We highly recommend TAMU ( TX A&M).

I am not a DVM......but I am married to one.