I will be originating at CMB flying on UL to BKK and transiting at BKK to Chiang Mai. I have a choice of airlines for the BKK-CNX flight - Bangkok Airways (90 min connection) or Thai Smile (3 hour connection). I will have separate tickets because I can not seem to find an on line ticket that will sell a business class ticket on Sri Lankan combined with a premium economy ticket on Bangkok Airways or Thai Smile. So I plan to buy tickets on each airline's web site.

Will Sri Lankan check my bags through to CNX in this situation with either airline (Bangkok Airways or Thai Smile) with two separate tickets?

Also, according to Expert Flyer, Sri Lankan has interline agreement with Bangkok Airways and Thai Airlines (TG). But no interline agreement is mentioned for Thai Smile (WE). If I purchase the Thai Smile ticket with a TG flight number would the answer regarding interlining be different?

I prefer taking the Thai Smile flight to allow for a longer connection time (and these are the last two flights of the night to CNX).

Thank you for any helpful guidance you can provide!