Hi there, I'm living in Samut Prakarn - I can't help with no.2 but for no.1 I'd suggest just trying another dealership. I've had my Aveo serviced at the dealership on Theperak (where I bought my car) and on Srinakarin - there was quite a difference in terms of service, helpfulness and time. For no.3, I was interested in LPG also, I PM'd another board member 'Phenomenon' who recommended a garage near Seacon, he also might know more garages that could help you.
Sorry to hear about your scrape, that sucks, near to Bang Na, you've got Imperial Sam Rong mall with quite good car-parts shops on the third floor - you could try there but they mainly sell the standard Honda, Toyota etc ones. I can't see any on ebay, I reckon you'd have to go to Chevy to get replacement lettering.