2nd blown head gasket, what gives?

at 272k miles my 2007 blew a head gasket. Plumes of white sweet smelling smoking coming out of the exhaust but I did not really notice the coolant level dropping nor was there milkshake in the oil. Since the engine was burning 1qt of oil every 1000 miles I decided it was time anyway.

I bought a junkyard engine with 125k miles and swapped it in the summer time. Genuine Toyota coolant was used. It drove fine and oil consumption was low. I put 7000 miles on it and over the last month I have noticed stronger and stronger coolant odor in the exhaust. Today after driving it harder for about 30 min and walking around the back of the car, I'm noticing again thick plumes of very steamy white exhaust, very strongly sweet like burning coolant. STILL no trace of milkshake in the oil and the engine coolant reservoir under the hood remains at medium. The car feels down in power.

Any idea what is going on? Why am I blowing so many head gaskets? I shudder to think of putting another engine into the Prius, this will probably be the kiss of death. Is there any other area of possible coolant intrusion into the intake? I know the throttle body has a small coolant line routed through it but I believe it is cast into the aluminum and impossible to leak at this area?

 

coolant level in rad is full up to the cap. Heat works fine, no to pump/thermostat was with the "new" engine. I did use bars leak on the last engine but thoroughly flushed the radiator and coolant recovery bottle with Prestone radiator flush and distilled water until no flakes were present.

light is on for p0420 due to using an aftermarket cat, also p0300, 301, 304 multiple misfire on cylinders 1,4. The car sputters at idle and idles rough. I changed plugs last week which helped for about one week, now they are likely fouled again.

Replace the HG would probably cost $1000 labor, no point on doing it on a vehicle with 280k miles on the body and the car is only worth about $1000 at this point at auction. Nevermind the host of other issues that could pop up soon like brake accumulator, traction battery, etc.

 

I have the torque app always on, this year I didn't tape my grill up and the engine barely gets to 160 with trips around town. Like I said I was very careful to flush through each hose so I know none are clogged, and I definitely burped every bit of air out of the system. It is dead of winter here and something like 25 degrees out. If the engine were to overheat it would have done so in the summer.

I am wondering if the blown HG is actually from oil consumption clogging up the cat and causing too much cylinder pressure.