I'll try to address all your questions...in no particular order.
I am currently 42yo, but did my first cleanse when I was about 30yo. At that time I had been diagnosed with systemic candidiasis, chronic fatigue syndrome and multiple chemical sensitivities. I was on a very strict candida diet, and using numerous alternative healing methods, supplements and herbs. I had made some significant progress in regaining my health, but had reached a plateau and researched the benefits of colon cleansing. I started the Holistic Horizons products and that is what produced the astounding results for me. I was still on the candida diet...protein and low carb whole foods. I stayed on this program for several months (4-6?) because the gunk just kept coming. I believe I weighed about 125lbs at the time, so I'm not a real big person either...but I would guess that I passed 10-20lbs of the stuff! My stomach was probably the flatest it has ever been when I finished that program. I believe the quantity of material was a reflection of a typical American diet that included a lot of processed and junk food, particularly in my teens and college years.
I have done a few different cleanses since that time, but without the dramatic results of the first cleanse. A few months ago I started using liquid Bentonite with whole psyllium husks and began to expel some of the same material as I had with the Holistic Horizons...although not the same quantity. I did it for a couple of months and then stopped...mainly because I got tired of taking the shakes. I don't have any particular regimine that I follow. I also started getting colonics pretty regularly a couple of years ago, so I never get to the point that I feel as toxic and congested as I once did. The colonics are a great addition to the herbal cleanses. I have done fasts from time to time, but usually not in conjuction with the colon cleanses. As I said in a previous post, I plan on doing the Holistic Horizon program again and adding liquid Bentonite to it...probably in the next few months.
I don't know if we ever get completely cleaned out. There are so many sources of toxins introduced in the food supply, water, air, chemicals...not to mention those created internally as a result of stress, and all of them create mucous in the body. Living in modern society means that we need to regularly undergo detoxification assisted by various cleanses.
If you are continuing to expel a lot of gunk, I would suggest that you continue with the psyllium and bentonite...but start eatting again. If you stick to simple, whole foods and keep your diet pretty clean with lots of vegetables, fruits, juices, some grains and protein you should still continue expelling. I would suggest that you switch to liquid bentonite, though. It is in suspension and doesn't draw as much fluid from your body.
I would also suggest that you begin dry skin brushing, if you don't already. It is a simple and quick procedure done daily that is very effective at cleansing the lymph and assisting with eliminating the toxic material. If you add this to your program you're liable to see an even greater quantity of gunk come out. You can usually find the brushes at health food stores...they are oval, natural brushes (sisal, I think) and you brush all of your dry skin towards your heart. This may help a lot with the swelling to which you referred. Daily skin brushing is great for keeping your skin soft, eliminates the need for body lotion, and helps to reduce cellulite.
As for parasites, everyone probably has them. The mucoid build-up is a breeding ground for them, so eliminating this material will help to eliminate the parasites. A parasite cleanse is also needed, but I believe that it is limited in its effectiveness if the colon remains full of toxic material. Remember that this material was not always solid and parasites may have taken hold within it prior to it hardening. Colon cleansing is the first line of defense in any detoxification program and will influence the effectiveness of any other cleansing performed. I would always recommend colon cleansing first, and then a parasite program...possibly overlapping the two.
This may surprise you, but given the odorous nature of the material you are passing you still have a ways to go. Putrefactive waste has a characteristic strong odor, but the older material is postputrefactive and has virtually no odor. It is harder and takes more effort to expel. My understanding is that the majority of this material resides in the large intestines, although mucous is also present in the small intestines and can become congested with toxins. There is a product made by Standard Process called Okra-Pepsin which is supposed to specifically address cleansing of the small intestines. I have yet to use it, but plan on including it as part of my next colon cleanse.
I highly recommend the booklet The Colon Health Handbook by Robert Gray. He developed the Holistic Horizons products, and this booklet describes his program, as well as giving a lot of valuable information about colon cleansing in general.