Coolant in the oil but did not overheat?

I have had this car for less than a week and have had zero issues that would raise any red flags or cause any major concerns prior to today. It passed smog without any issues, didn't see any noticeable leaks anywhere, haven't smelled anything funny or seen any strange smoke at all and the temperature gauge stayed consistently at just under the halfway mark at all times while driving. I couldn't even hear the motor when it was idling. However, I did start to hear a noise yesterday that I thought may have been a belt squeaking a little bit, but that's it.
Then this morning as I was driving to work, about 15 or so miles into the trip I suddenly started feeling a rough vibration that wasn't present before (mostly felt it by way of my feet, not so much by way of my hands on the wheel). About 3 or 4 miles later a solid engine light came on. Checked the temperature gauge and it was still good, it didn't smell anything weird or whatever either.
Between 2 to 5 miles after that, all of a sudden the vibration I felt became stronger and I not only felt it, but could now hear noise as well that matched the vibration, and at the same time the engine light began to blink. Almost instantaneously the car started losing power (or engine possibly shut down?) So, I turned on the hazards and pulled off to the side of the highway and turned off the ignition. Immediately I could see white smoke coming from under the hood even though the gauge never showed signs of overheating.
I popped the hood and then first looked under the car to see if I could see any indications of anything leaking or whatever and didn't really see anything obvious, then started checking under the hood stuff. First thing I noticed was some white stuff everywhere in the immediate area of the coolant reservoir which would indicate water (or coolant) having sprayed or whatever, and I could see that the coolant level was no longer at the level it had been after filling it the previous evening. In fact, there was nothing in the reservoir at all anymore! So, I grabbed the other 3/4 gallon I had and poured it in, which did not bring the level high enough to show inside the container. So, I looked under the car again to see if it was leaking out, but it wasn't. I knew it had to be going somewhere, so I pulled out the oil dip stick and found out where it went.
I was able to start the car and slowly move it to a safer location about 200 - 300 feet further up the road, and then had it towed somewhere after that.
My first instinct is blown head gasket. So, I am wondering what other possibilities there are and the steps to take for finding the culprit.
And also, if you have any recommendations on accessing very detailed step-by-step instructions for fixing any of those possibilities myself (including replacing the head gasket if that's what it turns out to be)? I am far from being an expert mechanic but am also not a stranger to working on a vehicle either. I actually replaced a head gasket in a Cavalier once before with a positive outcome so that's a plus!
Thank you in advance for any and all advice/instructions. I very much appreciate your time.