2001 Toyota 4Runner 2001 4Runner - Head gaskets and Injecto

Problems with my 4runner. Had the 90k service done. Replaced plugs (6), water pump, timing belt etc. A couple of weeks later it ran rough and the check engine light came on. I took it back to the mechanic and they said the injector wires shorted and needed to be replaced. It ran fine for a few weeks, but then I almost ran out of gas and it immediately started running very rough and the check engine light came on. I took to the same mechanic and they scanned and said there was a misfire code P0300 and that one of the fuel injectors was clogged so they replaced it. They indicated when you get real low on gas, the engine may suck up sediment on the bottom of the tank which can clog injectors etc. I used Techron as they said the other injectors might go bad. Three weeks later, same problem, rough running, check engine light, same as the two times before. This time they scanned and another misfire code of P0300 so another injector was bad. They had to tear into the engine to get at it, expensive, and we agreed they would replace the other two injectors on that side at the same time since they had access to it and it seemed like the injectors had issues. Thought I would save money in the event the other 2 went out in the future. The bad injector seemed to have been fried from running hot.

Three days later, the exact issues came up again. This time they finally determined the head gaskets were bad. There was leaking of coolant which crystalized and then acted a s a sand blaster, damaging the plugs. Lots of carbon in the engine etc. The truck had not been running hot.

They said the timing belt was fine. They specifically checked. They said if it was a timing belt issue it would have been very sluggish. After calling numerous shops and Toyota, they determined it was a gasket leak that was between No. 1 and No. 3 which led to the coolant being in the engine which led to all of the carbon. They resurfaced the heads. The valves were OK so they don’t think the issues were caused by the timing belt. They did adjust the valves. They also replaced all of the plugs.

Seems like the initial 90k service may have led to all the problems since they were dealing with the water pump etc. Seems like they may have simply misdiagnosed the injector issue (spark plug wire issue?) Twice, but that was about $1,200 worth of misdiagnoses. Odd that it ran rough immediately after almost running out of gas.

Seems like at best, they misdiagnosed the problems 2-3 times and at worst, their original work caused the issues. Common for a 2001 Toyota 4 Runner to have bad gaskets, injectors? Is there any way I can legitimately establish any of that? What do you think? Just a series of problems that all started right after the 90k service.