Seeking advice for likely head gasket issue

Short version: If the startup rattle / check engine light came on 3 drives ago, (i.e. I've started the car three times and driven it since the first start rattle) and no other symptoms are obvious (coolant fluid, leaks, oil level/cleanliness) is it likely ok to drive a bit more (1-5 times) without causing lasting damage to the engine?

Long version: 2013 model 2 with 202k on it. I got a check engine light 4 months ago that coded for cylinder three misfire. As it was at 195k at the time, I went ahead and replaced all 4 plugs and coils. No check engine light or performance issues between then and last week when the startup knocks began. After the issue started, I drove it twice more, read many threads here, got the codes and haven't driven it since. It showed cylinder 2 and 4 misfires and multiple cylinder misfire codes. (P302,P304, & P300)

Once it warms up, it doesn't knock, which leads me to believe it is a head gasket leak. I'm ordering a combustion leak detection kit to confirm before moving on any repairs, DIY or real expensive.

The reason for this post is as above, I have need to drive it a few more times before I can realistically work on it as our second Prius (my wife's) is needed by her once in a while. What is the gut reaction to those of you who have more experience with such things on whether or not I can "safely" get away with driving it a bit more? I fully understand that any opinions offered are no guarantee of anything and things can go wrong at any time. That said, I'm looking for group opinion on the matter.

Thanks in advance.

 

My round trip is 80 miles, or if I let my wife drive the ticking time bomb, about 15 miles.
No smoke or other signs present.

 

The test is definitely for combustion gases in cooling system. The video that drove me there made the point that if it is leaking while at rest, while under combustive pressure, the leak should go the other direction, which will drive the gases into the cooling system, which is why this test is a "definitely your head gasket" kind of test. The logic sounded reasonable. While I understand the boroscope inspection would be ideal, that is a more expensive tool that I don't have and would have very little use for regularly.

As for the EGR system, I didn't know anything about it until this happened and I read all your threads. Your signature page is very helpful by the way. So to answer your question, never. Given the symptoms and age, I'm hypothesizing the gasket is leaking and the gas test will confirm. (I'll know this weekend) I'm sure the EGR will also be dirty at best and near blocked at worst, but if I need the gasket replaced, the EGR cleaning will just be along for the ride.