Not a mechanic but I know some newer cars do require premium gas and if you don’t use it your car without the knock sensor would start knocking but since cars are so smart they will just adjust the timing and that won’t happen but I am sure they will detect the issue. So by putting in the correct gas that engine light should go away but it might take a couple fills before the old gas is diluted enough and it goes away. It sounds like it did go away for a bit which is a good sign but probably a bit was still left in the tank and the sensor detected it again. Get the right gas and give it more time 2 mechanics told you the same thing so probably safe bet it is correct. Lastly get yourself a code reader. They are like 10 - 20 bucks and next time you come here will get way better responses as to your problem. Good luck
If it was me I would take that as a huge warning sign that the seller probably did his best to hide the fact that the transmission was going and that is why they sold it. And you bought it. Honestly I would sell it to the next sucker and not be the one holding the bag but that is a shitty thing to do so you weigh in on your conscience and decide if you can do that, you might be screwing someone over. Last time I bought a shit car the trans had a slight issue between 2nd and 3rd where it would seem to get confused and flutter between the 2 gears. It was better to put it into manual shift and it would stay in the gear you told it to. I drove it a year and sold it with full disclosure about the shit gear issue. Made more money then I bought it for and it was at that point I vowed to never buy another dodge jeep chrysler vehicle, fuck even in zombiepocolypse I am not taking a dodge brand vehicle if that is all there is.