To answer your question regarding an excision, yes, it can be done even if the tumor is very near the sphincter. HOWEVER, with a tumor the size that you are mentioning, an excision is almost out of the question unless they can shrink it first. When they do an excision, they will do a full thickness....meaning that they would go down and through the rectal wall. The hole is either closed with sutures or, in some cases of small 'holes' (depending on size and location) a mesh patch of sorts can be sewn in. Normally, the rule of thumb with an excision is to have a tumor that is less than 40% of the circumference of the rectum. Think of it this way. Visualize the leg of a pair of pants. You get a hole in the leg...and NOT at the seam. You either 'patch' that hole so that the circumference of the pant leg remains the same OR you sew up the hole. IF, however, the hole is too big then you are making the leg smaller by stitching the hole up. If it is a small hole....likely you won't notice any difference. A larger hole would lead to the pant leg being smaller and likely more uncomfortable. Now, take that analogy and think of what would happen if you made your rectum significantly smaller. Your body would have a very difficult time evacuating the fecal matter causing more problems for you. In cases where patients have excisions (either TEM or just an excision), the tumor is generally small and/or the patient has undergone radiation to shrink the tumor.
It sounds as though you're in a very difficult situation. If the tumor is that large I'm honestly not sure that you have a lot of options without having them attempt to first shrink it...and then comes the difficulty of having radiation on a tumor that doesn't test positive for cancer. I would also like to address another issue in your post. You assume that APR gives you a guarantee of no recurrence but that isn't true as well. IF..big IF...there are cancer cells associated with this tumor, then there is NO guarantee with whatever surgery you have. Better chances, yes....but NO guarantees. That is a fallacy to assume that if someone has an APR there is a guarantee that cancer won't return. IF your tumor truly has no cancerous cells, then you wouldn't be facing (at least from this tumor) any possibility with an excision of mets.
There are some small studies regarding an artificial sphincter but from what I remember reading, they are used on patients who have fecal incontinence NOT due to surgery and are used as a last resort before having stoma surgery. Ten years ago (and I recognize things have and will continue to change), the attitude was simple. Why would there be a need for more research into avoiding a colostomy for rectal cancer patients....you can live a full life with one (and I'm not saying you can't) so why bother doing research into something that truly isn't "necessary". Personally, I don't agree with that assessment but I do believe that that attitude still prevails today.
I'm rather curious that with a tumor that size and encompassing almost your entire rectum, cancer or not, aren't the surgeons concerned that you could get a blockage because of this? Aren't they pushing for some surgery ASAP to relieve any potential problems? As for the CEA, you are understanding correctly. My CEA was 0.7 at diagnosis with a very large rectal tumor. After chemoradiation, surgery and more chemo, my CEA remains in the 0.9 - 1.7 range (it can and does fluctuate) so the test is NOT a good indicator for me. I could have a recurrence and likely not know about it based solely on that test. We stopped running the test....it just doesn't make sense and only adds more cost to my bill.
If I were in your position, considering that it appears you have few options due to the size and location of the tumor, I might consider asking the doctors about radiation, if for no other reason than to attempt to shrink the tumor for removal by TEM. The problems with that option are: (1) putting a patient through radiation without a diagnosis of cancer runs risks as well, (2) insurance may not pay for the treatment which is VERY costly and (3) if you had radiation and the treatments did their job, then you run the risk of never knowing whether or not the tumor contained any cancer cells.
Are you 100% certain that the surgeons you are seeking advice from are board certified in COLON and RECTAL surgery. Too many General surgeons (also board certified but just in general surgery) will claim to be 'experts' in this field and while they certainly can do the surgery, they are not board certified in COLON and RECTAL surgery as well as general surgery. Here is the link to verify that the surgeon(s) that you are seeking advice from are certified.
http://www.abcrs.org/verify-a-physician-2/Please realize that even though you may have gone to a cancer center....many times those surgeons are just certified in general surgery. It is at least something to think about. Good luck.