Can we use the visa exemption + extension for a 71-day stay in Thailand?

45 days plus 30 days ! Job done👍

No problemo, 45 day, then extend for 30.

If you're worried about onward ticket proof. Just buy a cheap bus ticket out

To be absolutely safe, you should book a quick flight to a nearby country like Malaysia, Vietnam, or Cambodia etc. Heck, take the 3 day excursion. It will save you from having to go get an extension.

Some people use a service to book a "throwaway" ticket.

Some people will press their luck, and if their airline denies them boarding, they will quickly go online and book a flight. (Luck of the draw on the prices you will find). Thai immigration is not as easily satisfied I don't think.

It’s now 45 days on arrival then you can extend at immigration another 45 days

I think as long as you have a few pound ,anything goes

You get 45 days on entry with visa on arrival and you can add 30 days at the end of the 45.

You'll be fine. The visa exempt entry stamp is now for 45 days until the end of March, so your one extension will get you to your 71 days. As long as your return ticket is with the same airline you should have no problems.

You should be OK with either. The airline are the ones who might create an issue with either visa option.

This happened to a friend recently so best advice is to book a throw away/onward ticket within the time frame so that you can show you plan to leave Thailand (even though you won't use it)

Airline might create an issue, possibly requiring an outward tix within the terms of your entry.

Please check with your airline.