Coupla different things. Rshtuni may be able to command an element of support among the Armenian portions of the army. The dynasty is Armenian as well after all and they may have connexions. Yennadios in Africa will probably continue with nominal loyalty to whatever government gets set up around Konstantinos IV at Constantinople (the Regents will probably be Fausta, his mother, who has some knowledge of intrigue due to the circumstances surrounding her father Valentinos' deposition of Heraklonas; Maurianos, Rshtuni's old opponent; and some other military officers and bureaucrats). As soon as any civil war starts in earnest, though, he will break off. He's got plenty of cash on hand and thus will be able to survive on his own in southern Italy, Sicily, and North Africa quite ably. In addition, he will be able to take in losing armies from the civil war to boost his manpower, further improving his odds at repelling the Arabs.
You can pretty much fashion the course of the civil war however you like. This particular period of history is not well documented, and the motivations of many of the players subordinate to Konstas II at this time IOTL are not very clear. Konstas II's brother Theodosios was executed IOTL 661 on possibly spurious charges of conspiracy, and it's up in the air whether he entertained designs on the throne for himself. He's a player, and could side with either the regency or whatever Rshtuni and the rest of the pro-Arab party can get together. Nikephoros Patrikios, a competent but not brilliant military commander, was largely loyal to the line of Herakleios and would probably side with the regency council. Then, of course, there are the Arabs, led by their military commander (but not
khalifa yet) Mu'awiyah, who himself is quite able. They are probably the strongest of any of the competing groups, but also the most dispersed, the most prone to revolts, and the most distracted. They will also need to deal with the fallout of Uthman's death whenever it comes ITTL, because 'Ali and Mu'awiyah will probably still end up in their showdown for mastery of the
ummah.
Hope that helps somewhat, sorry for not responding sooner.