In 2005 my dad had triple bypass surgery. He also had rhythm problems after surgery.
I am not aware of any relationship between bypass surgery and the development of a heart arrhythmia - but his heart doctors should know and let you know.
Arrhythmia is caused by "electrical" signal generation and transmission problems inside the heart and its muscles ... which are not to my knowledge "touched" in bypass surgery.
If the medicare issue is you don't have insurance coverage, my experience says one should be able put off any surgery, including ablation procedures, for several months. It would likely require the application of some medications to control the severity of the rhythm problems/symptoms and to reduce the possibility of clot formation. All of that said, there is no 'magic bullet" to cure arrhythmia problems...so unless your bypass surgeon knows there is something done during his surgery that can be corrected/modified/whatever, I would not be over confident that "going back in" will fix the problem.
I suffer from AFib and have undergone open heart surgery for a leaky heart valve. While open I underwent a "maze" procedure to cure AFib, it did not stop the AFib. My valve surgeon (maze surgeon) gave the maze procedure about a 60% chance of success, I came in the 40% side.