Many years ago I had a roommate who had a little Civic (the early ones) and the brakes were in need of replacement. I offered to "give him a hand" with them and didn't really understand his reluctance to approach this basic task...and then he shared this story.

He wanted to change the oil, everyone told him how simple it was. He had some basic tools, so he purchased a drain pan, filter, filter wrench and oil, and slid under the car. He removed the drain plug and let it drain, and then put the plug back in. He changed the filter. Then he added the fresh oil. It looked a little high on the dipstick but his friends had told him it would be a little high until he'd run it to circulate the new oil.

It cranked a little funny, started okay, and as it started to warm up, it just sounded funny, so he revved it a little and the engine quit, with smoke pouring from under the hood. He opened the hood and there was oil around in various places and he couldn't tell where it came from, so he tried starting it again and it just clunked.

It turned out that he'd actually drained the transmission fluid, and then of course overfilled the engine; the theory was that when he revved it it was enough to have the oil level just high enough for a descending piston skirt -- and you can't compress fluids. While the engine was pretty clearly toast, they also assumed the transmission was in pretty bad shape since it had been run on very little fluid. In any event, it was an older car anyway, and this was it's swan song on the way to the boneyard.

That's when he bought the Civic.

Then I understood his reluctance.

Good luck with the replacement engine, R Leo. It certainly sucks, but in the end, it's just some time and money. No one got hurt, there's no long-lasting damage, and you sound as though you're able to grin at yourself, albeit wryly.