Head gasket and coolant flow problem

Need some help and advice.

I have a 1999 Deville, ran great but had a small coolant leak.
Never saw puddle but could see coolant on radiator passenger side and used bars for 2 years had to top over once in a while. Never thought it to be a big problem. Had good flow through rad hoses and rad. Had to top off coolant once in a while. No big deal.

Car overheated recently. Never did this before. Had good flow through rad hoses and rad. Assumed it was due to slightly low coolant level. Topped it off and drove it for a couple days.

Then very shortly after, belt broke on water pump and car overheated badly.

Had radiator and belt replaced. But since then no coolant flow to radiator. Had thermostat replaced and did not help. Top rad hose only had air. No coolant flow. Car now (since overheating) has certain head gasket problem as I can see smoke in the exhaust and smell combustion gas in the coolant reservior. In hindsight, gasket was probably going for some time.

Cant drive it at all as it overheats in 5 min. After rad replaced and therostat replaced, I tried and removed thermostat but I still have no coolant flow at all. I checked purge line and it is clear. When I turn on car, purge line splits water when line is disconnected. If I overfill reservoir it flows steady. So I assume clear. Rad hoses get (top and bottom) hot but radiator is ice old. Let car get to 210F and shut off. Tried flushing with rad cap off, on, and with new cap all with same result. With car off, flushed with water from reservoir to upper hose and plugged and flushed to lower hose and pluffed and flused out purge. Flushed from upper to lower and from lower to upper to confirm no blockages.

Confirmed belt for water pump is spinning. Not sure if pump is working though?

I was going to try and use Duraseal or Blue Devil to seal head gasket but am concerned I have no coolant flow through the engine to the radiator at all.

Car repair shop says it is the head gasket and could be $2500 to $4000 to fix if both heads and bad. It is more than I want to spend on the car versus replacement.

Would a head gasket leak immediately vapor lock the water pump? Or is the water pump bad too? Any experience with Blue devil or dura seal?