Sorry for the delay getting back, work and funerals got in the way. It did give me time to have the truck towed in to the dealer again for some diagnostics, then towed home after.
Third owner. No mods. No flooding. Talked with both previous owners, and the dealership that did most of the repairs and maintenance. First owner said he got rid of it after the engine headaches (he said the engine was replaced, couldn't remember if it was under the warranty or just after that expired) and problems with the fuel system. Second owner had it for about 5k and reported no problems.
Been to the dealer 7 times (well, 3x to one dealer, 4 times to another) on the fuel problem. Basically the sensor/computer doesn't correctly read the alcohol content of the fuel. The dealer resets it, it resets itself. Right now the computer thinks it is running on 80% alcohol; a bunch of components were replaced, the tank was flushed (again), and the truck filled with straight gasoline about 200 miles before it died last time. So far on this tank of fuel, I've averaged just over 8 mpg. When the flex-fuel system decides to behave, I get about 17 mpg.
As for the engine, one dealer said to buy a new truck. The service writer at the other suggested repairs to the tune of $4k, then looked at the service records and rather sheepishly said "oops" as it has had all of his recommended repairs and upgrades. His next suggestion was to replace the engine with a shiny new LC9. I might have considered it if he hadn't added, "most of the problems are fixed now".
The independent mechanics I've dealt with the most used to run a shop for one of the area dealers (before that dealer got shut down in the Government Motors fiasco). He did some diagnostics but refused to try to fix it because he doesn't feel he can guarantee his work on an LC9, and thinks the engine is to far gone to be re-buildable for less than the cost of a different engine. (First time I've ever stood there with cash in hand and had a mechanic tell me that.) Back when I first started having problems with it in December '16, he suggested replacing the engine with something other than an LC9 and getting rid of the flex-fuel system, but admitted he didn't know if it could be done. He thought maybe someone in the Chevy Avalanche Fan Club might know, gave me a flyer he had sitting on his desk, and suggested joining to find out.
I've talked with 4 other independent mechanics who were willing to work on it (most of the other mechanics I know specialize in older vehicles). Two suggested junking it, one said the only way he'd work on it was if I wanted to convert to diesel, and one suggested driving it into a bridge embankment.
I don't want a diesel, and of course I won't drive it into a bridge embankment: it isn't running. If it isn't possible to fix the problems, I'll part it out or scrap it. After almost a year of dealing with it, I just want to make sure I've considered every reasonable possibility.