Toilet replacement-possible disaster

Recently, I had to go back south to the town where I was born and raised for the funeral of a friend, you can read about it here
I stayed two nights with a Nephew, who is 3 yrs older than me, the top image is his home. Very nice.
When I rode up, there was a toilet sitting on the grass next to their driveway.

When I asked about this oddity, they said it was a perfectly fine, 30 yr old Kohler, but it was rather low, making it not so good for folks with bad knees or other handicaps.
I asked, they were going to get rid of it, so I brought it home.
That was over a month ago.
Yesterday, I finally got around to DOING something about it.
But first, since I suspected there was more slipshod workmanship here in the "new bathroom" (since they installed a USED toilet in the first place, complete with years of rust in the tank and corresponding years of mineral build up under the rim) I decided to go all out and check under that floor, so meet "Plumber Jerry"

I did NOT want any spiders or other vermin, nor dust or insulation in my hair or down my back 🤣

The condition of the flooring seems alright, I do not understand what that rectangular cut out is all about. I now suspect that the idiots that screwed up so much else, perhaps cut that thinking the "Terlet" would go there?
Since I had the wax ring, the new seals for the tank/bowl connection, I removed the old toilet

and set it down in the shower stall.
Folks, this is a VERY small bathroom.

I scraped out the old wax ring (that stuff is SOOO sticky, I hate messing with it) and the bolts for securing the toilet: I had new ones.
I brought in my new/old "Throne"

And proceeded to install the seals for the tank to the bowl. Out here I had room to work, but it was still a BEAR.

This was my first time messing with a "Three Bolt" tank, in my past experiences, there were two bolts securing the tank to the bowl.
So the seals (see in the ↑photo, circled↑) were one piece, three rubbery places to insert through the porcelain for the bolts.
The new one did not fit well.
While assembling the 'new/old' replacement, the gasket between tank and bowl wasn't going in as easily as I thought it should, and while I was twisting to get the holes to line up better, I heard a ¡PING!, looked and saw a hairline crack, that for all I know might have been there already.
So with it still sitting on my flatbed cart, I rolled it back outside, put about a gallon of water in the tank, waiting to see if there is any leakage.
After an hour, no leaks, but I'm afraid when I go to try and get it seated, it might make the crack spread.
One of my friends commented:

When you're seated on a toilet, a crack usually does spread

I went on and commented back about things to do and hoping for the best.
It was over 24 hrs before it sank in, he had made a joke!

^{↑I have no earthly idea who made that image↑}
So after all that, from the time I came in from checking underneath and beginning on the project, and hour had past while I removed the old, got the opening ready to receive and was working on the assembly, when the problem with the crack sidelined my progress.
Still and all, I give you my new/old throne:

The board covering the opening to under my bathroom (which also has the dryer vent and water heater overflow tube,) had some rot across the bottom, so I added an old piece of 2x4, screwed it all together

So that now, I am hoping no leaks appear.

And there you have my

"DIY Toilet Replacement"

Toilet replacement-possible disaster | Ecency