Re DMPS. Remember when considering a person's experience with a chelator it is critical to know what dosing protocol they were using. Many of the worst experiences with DMPS have been with high dose IV. dmpsbackfire.com has many reports.
In Frequent Dose Chelation group--3000 members/ 1700 posts/month--DMPS is very clearly better tolerated in very sick people like you who have gut problems. Of course FDC group provides support for those using Cutler's protocol. These people are typically taking 10mg doses of DMPS every 6 or 8 hours along with grams of vitamin c, some vit E, Mg , Zn etc.
Eventually you'd add ALA to help pull Hg and As out of organs and the brain. Most of the ALA will go out via bile but I understand you have problems there. However since you'd also be using DMPS, it can also go out via urine. That is one of the nice aspects of Cutler's system.