Occasional death rattle on startup – but not during day – chances engine dies during day?

My question is: Assuming my engine does not destroy itself during the death rattle in the morning, how likely is it to do so during the day?

This might be an odd question because I am wondering when (time of day) my engine is going to fail as opposed to how to diagnose / fix something. My 2011 Prius with 198k miles sometimes has a minor death rattle on cold start in the morning. It is a very minor rattle and resolves itself within 5 seconds. Car runs fine for the rest of the day, until the next morning, when the cycle repeats. Yes, it appears to be burning coolant, so there is likely a head gasket issue. It takes a little over a year for the coolant to go from full to low (supposedly .5 liters) which in my mind is a very small leak. For purposes of this question, just assume there is a small head gasket leak.

The death rattle only occurs in the morning. My typical drive cycle is that my drive to work is long enough that the engine comes up to normal temperature. Some days I take a short trip after the engine has cooled to get lunch, where the engine does not get up to temp. Other days I don’t get lunch so the engine stays cool for the full work day until I go home. My drive home gets the engine back up to normal temp. The only time the death rattle ever occurs is in the morning after an overnight cold soak.

If the engine blows up in my driveway at home, I am perfectly ok with that. I will just get in my other car and go about my day. If the engine blows up while I am at work/driving, it is a much bigger problem. First, I park in an underground parking garage in a high rise building in Chicago. Getting a tow truck in there is difficult and expensive. Second, if my engine blows up in the underground garage, they are going to be upset about 5 quarts of oil being dumped on their clean garage floor. I am sure they will charge me a huge cleanup/environmental fee. Third, if the engine blows up while I am driving on the road, I may block traffic on a major highway, inconveniencing an uncountable number of people.

To anticipate some questions / comments:

1. I am aware of the multiple techniques for confirming head gasket leaks. The majority involve some level of disassembly of the engine compartment (wipers, cowl, etc.). It is November and I am in the Chicagoland area. It is cold and my only place to work on the vehicle is outside. I am not doing that until next May at earliest

2. I don’t want to pay for repairs. Local Toyota dealer quoted $5176.00 for head gasket replacement. Another chain shop was $3600.00+. I can’t imagine an independent mechanic would be less than $2500.00. I feel I have gotten my full value out of this car and don’t want to put more money into it.

3. Some are going to say the prices quoted for repair are too high. Although I might agree with that statement, these are the prices where I live. I don’t necessarily live where you live.