Hello and welcome Muse! Sounds like you have done better than many at dealing with your JW family.
I never was a JW but I did have colon cancer 5 years ago. My polyp was not benign, I had about 18 inches of colon removed, and all went well.
If your father has a good surgeon, the procedure should go just fine. It is not a fun process, but it is not as life-threatening as you may envision. Be strong, have Faith in God that he will pull your dad through, and that He will provide some peace for you too! I mainly want you to know you may be over-stressing, which I myself tend to do! So, please, take it easy on yourself.
I wish you both well, please keep us posted.
AJ