2013 Prius Misfire

My grand daughter has a 2013 Prius with about 190k miles. It has recently started experiencing the idle rattle often associated with head gasket failures and/or EGR issues. Originally when the problem started there were no codes but then there was a misfire code (generic). I replaced the spark plugs (they appeared to be original) and it was okay for a short time. The rattle returned and the check engine light came on with codes for random misfire on cylinders 1, 2, and 4. I cleaned the intake manifold and EGR tube - the tube was not too bad but the EGR ports in the manifold were basically plugged. There was a pool of oil-water (?) mix in the bottom of the intake when the throttle body was removed. I replaced the PSV valve while the intake was off. After the intake was reinstalled the rattle returned so I am looking for alternative corrective actions to consider. I should acknowledge that there is some coolant loss but no indication where the coolant is going. The original spark plugs didn't show any indicators and other than the potential of coolant mixing with the oil in the intake it's not apparent where the coolant is going (it isn't going fast).
The head gasket seems the next likely problem area but I don't understand how a head gasket failure would affect 3 of the 4 cylinders so I don't want to jump down that rabbit hole until I have a better sense of potential success.
I do not believe that the engine has ever over heated but reporting of those type of details can be sketchy from my grand daughter.
I thought about the potential that the coolant passing through the throttle body may be a path of leakage that would affect multiple cylinders but I haven't found any information regarding this potential failure so it seems remote.
I would appreciate any guidance from the community as I need to decide on an action plan that can be executed over a short time window as it is her only vehicle for getting to and from work.