Will I face issues upon arriving in Thailand with a long-term Non-O multiple entry visa if my return ticket is for five months later?

You should not encounter any problems when arriving in Thailand with a Non-Immigrant Type O multiple entry visa, even if your return ticket is for five months later. The multiple entry visa allows you to exit and re-enter the country, restarting the 90-day reporting requirement upon your return. Proof of onward travel is not necessary for valid long-term visa holders.