Can a Nepali passport holder transit through Thailand without a visa on a same-day flight?

I have a Nepali client traveling with an original itinerary of Thailand (3 nights), Singapore (2 nights), and Malaysia (3 nights). Singapore is visa on arrival for Nepali passport holders, while Thailand and Malaysia require pre-applied visas. All tickets were issued for visa purposes; however, the Thailand visa was rejected on 23 December and marked under a “watch list,” meaning re-application is not possible for at least one month, while the Malaysia visa is already received. The issued tickets are Nepal–Thailand and Thailand–Singapore with AirAsia, Singapore–Malaysia by coach, and Malaysia–Nepal with Batik Air. Since Nepal–Singapore airfare is currently very expensive, we are considering using the existing Nepal–Thailand ticket only for transit and changing the Thailand–Singapore flight to the same day, keeping the passenger airside without clearing Thai immigration and traveling with hand carry only (no checked baggage). Another option is to purchase a new Thailand–Singapore ticket while retaining or rescheduling the remaining sectors to reduce cost. I would appreciate guidance from fellow agents and operators on whether a Nepali passport holder can transit through Thailand without a Thailand visa under these conditions (same-day transit, airside only, no baggage, no immigration clearance, different PNR ticket) and whether airlines allow boarding in such cases?