re: "With coffee, it can only be worse since coffee usually keeps me wired."
Coffee in the sigmoid colon should not make your body feel the effects of the caffein whatsoever. In my experience it never has.
re: "I read that if you hold an enema for more than 15min, it can cause systemic absorption. Is that possible with only one qt of solution which is not enough to move beyond the lower colon?"
You should know that you only want to have a pint at a time inside you, a pint for about 15 minutes, release it, then bring the 2nd pint in. Total of a quart, but in 2 separate half-quart segments. And i believe i have read that you can hold the pint of coffee inside for longer than 15 minutes, but i will look for somebody else to verify this.