First, enemas backing up into the small intestine...
It happens fairly often for me. My large intestine holds about 2.5 - 3 quarts. Over that, I can feel the solution working its way back through the many twists and turns of the small intestine. I occasionally do several 4-5 quart enemas in a week with no ill effects.
Second, an enema backfilling into your stomach...
I believe it can happen, and the results are not at all fun. A cautionary real life tale (not for those with queasy stomachs)...
I once hit a *major* bout of depression, curled up in bed and hid from the world. I just didn't feel like eating, and what little I did eat was whatever I had left over in the fridge, sometimes on the verge of going rancid. One day I woke up feeling a little bloated and gassy. I had successfully treated that feeling before with an enema so I decided to just take a large enema until I felt full to flush out whatever might be bothering me. I took the first bag (2 qts) with no problem at all. The second bag also went in with no discomfort. The third bag made me feel a little heavy, but not uncomfortable. I got about 1/3 of the 4th bag in when I started to feel intense pressure. I stopped at about half way on the 4th bag and stepped on the bathroom scale. ... then it hit. Like Jimmyt44 described... nausea like I've never felt before, belches that smelled and tasted like feces, nearly uncontrollable vomiting of water and fecal matter for several minutes, then cramps and explosive expulsion.
As I sat there emptying my body from both ends, I realized I had not eaten for 4 days so my bowels were almost completely empty before the enema and suspected I had flushed fecal matter back into my stomach. When I finished expelling the enema, I stepped on the scale again... 12.5 lbs lighter! That's 6.5 quarts of water! I later found a medical book that said the average adult human intestine has a maximum capacity of 7 quarts, supporting my hypothesis.
What happened after that? My body had absorbed too much water from the enema, making me hypernatremic. My whole body swelled, I dripped with sweat over my whole body, I had a screaming headache, felt nauseated, and my breath smelled and tasted like feces. I drank some apple and orange juice, knowing my kidneys would work extra hard to flush out the added sugar and remove some of the water from my system. An hour or 2 later I started grazing on a light meal (real food!) chased with Gatorade for electrolytes.
It was almost 4 days before I felt anywhere near normal and almost 2 weeks before my bowels worked normally. Some time later I told a friend who's an ER nurse what had happened and she said I was lucky that I lived. That much water absorbed so quickly definitely threw off my electrolytes, risking cardiac arrest, and the backwash of fecal matter had upset my intestinal flora, affecting my body's absorption of nutrients.
Is it possible to get water into the stomach from an enema? I'd say yes. Should you try it? I would say not only NO, but @#$%^& NO!!!