A Center Controller reported a flight of two RV8's descended from their assigned altitude and flew below the Minimum IFR Altitude. Later in the flight the same aircraft deviated from their assigned route.

Narrative

Aircraft X; a flight of 2/RV8; was on an IFR flight plan to ZZZ at 9000 ft. through Sector ZZZ and ZZZ1 sector respectively. The aircraft requested lower and was descended to 5000 ft. then to 3500 ft. per the MIA (Minimum IFR Altitude) in that area. After passing the 3500 ft. MIA I witnessed the aircraft begin to descend on his own. Out of 3300 ft. I told Aircraft X to maintain 3000 ft. and that the minimum IFR altitude in that area was 2800 ft. I witnessed the aircraft continue to descend through 2800 ft. at which point I gave a Low Altitude Alert and told the aircraft to climb back to 2800 ft. The aircraft responded and began to climb. Once at the appropriate altitude the aircraft advised he would no longer like to conduct a visual approach and wanted vectors for the RNAV XX instead. For clarification; the aircraft did not have an approach clearance at any time. Person A pointed the aircraft out to ZZZ approach and got control; as well as pointing the aircraft out to sector ZZZ2 and got control. I then cleared the aircraft to the initial approach fix ZZZZZ. After about a minute I noticed the aircraft was about 15 degrees left of course from his route line that showed direct ZZZZZ. I advised the aircraft he was left of course and cleared the aircraft back to ZZZZZ; at which point the aircraft asked for vectors for the RNAV XX. I told the aircraft we cannot provide vectors for that approach as it is not depicted on our radar scope and clarified that if the aircraft wanted vectors to load the fix into the system that I could do that but not vectors for the approach. The aircraft then stated that he had 'found a hole in the weather' and wanted to cancel IFR; which he did. I first reported this event to the Front Line Manager (FLM) when the aircraft descended below the MIA; then reported again when the aircraft was not direct ZZZZZ.