I have blow many transmissions and my Bonneville transaxle. On my Bonneville Transaxle drive was just a sound but 1st still worked. Yes shifting can be a problem caused by other issues but a good mechanic first does a proper diagnostic and systematically rules out these other issues so instead of hypothesizing he just says I have ruled out all the other issues and this is what is wrong. So if he can prove the fuel is one of the problems great let him. Then see what happens. Just make sure he can prove that fuel is the problem. Also if he tries to take you for a "$" ride make sure you let him know that he is paying if he is wrong. So many mechanics just do the "swap out parts until its fixed” routine and make the customer pay for it. A good mechanic will do a good diagnostic and explain why he is sure the fuel is broken then he will fix it. There is usually a "good" reason for just replacing parts instead of doing a deeper diagnostic and it usually is the cost difference is prohibitive. In this case I would say the cost to replace all plausible causes of transaxle problems is much more expensive than a good diagnostic of the systems involved. In some cases you may have many problems but i have never had a problem that could not be fixed or diagnosed without a friendly diagnostic and a bit if testing. Testing pressure and showing you a reading will work so will showing there is a proper current to the fuel line. All easy tests’ that can help you trust what the guy is saying. If he can’t back it up with a test then it’* a risk, and you are the one paying. Remember to tell him its business not personal.