To answer your question as asked, based on the mechaber in 261:4 you may not keep food with a shehiya issue on the "fire" from the time you accept Shabbos. Consider the following instead:
The minhag of most in America is to allow shehiya if the food is cooked 1/3 or 1/2 (ask your rav). Many are not lenient (again- ask your Rav).
Placing aluminum foil on the inner shell/element (and acc. to some the adjustable knob) would serve as a blech and a hekir not to raise the flame. This would allow you to do shehiya no matter how cooked the food is.
(The raw meat trick in 251:3 won't work if you are going to shul for mincha and kabbalas shabbos before actually being mekabel shabbos.)
If you came home from shul and realized you made a mistake, you can eat the chulent. Firstly, you have the gra quoted by the first biur halacha in 318 that an unintentional derabbonon is mutar. Secondly, the biur halacha in 253:1 "lehashhoso" allows you to occasionally rely bedieved on those who permit shehiya if his intention is to eat it the next day (different than other areas where you can always rely bedieved on a dissenting opinion).