An SETV and exit ticket to Vietnam within 60 days would be the most hassle-free for you. No visit to Immigration for extension and no hoops to jump with airline and passport control.
If you have a visa, the airline may not even care about your return flight since you are less likely to be denied entry at passport. It then falls on passport control to decide if your return flight is reasonable, if asked to show.
Yes, 2 months before, 3 months after (and no less than 400k until the next extension). Tod's suggestion about getting a new Non-O visa gets the longest temporary solution, but it will depend on him willing to leave her side for 2 days.
When are you going to Savannakhet to get the new visa? When you stamp out of Thailand, you can show the entry stamp from the old passport and they should stamp you out on the new one.
Return ticket outside of 60 days with no visa isn’t much better than a one way ticket. The airline is making a determination if Thai Immigration will reject your entry and they have to carry you back.
In other words, nothing you’ve described is a red flag 🚩 for visa denial or entry denial. BUT embassy/consulate officers and Immigration officers have some discretion to approve or not approve.
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