Is it acceptable to have a return ticket that reflects a 45-day stay if entering Thailand on a visa exemption and planning an extension?

Airlines and immigration are both aware that many people will extend, and you will find that on onward (not necessarily return) ticket outside of the 45 days, but within the 75 days will be accepted.

Stan's comment above totally missed the question and lectured you on verbiage. To answer your question, your plan is fine !! A return ticket is required but the date is generally NOT scrutinized.

You don't get a 'visa on arrival (with a visa exemption)'. That's an oxymoron.

You can't have both at the same time and as you appear to be from one of the eligible countries , you will enter on a visa **exemption**. That means you are EXEMPT from needing a visa to enter the country, so how can you also have a 'visa on arrival'??

Those are only issued to a very few countries and are for 15 days only. You would enter on a visa exemption of (currently) 45 days, pre-Covid 30 days.

Then you can extend the VE for another 30 days so your return ticket would need to accommodate this, if that is your plan.

The attached chart (with countries who need a Visa on Arrival highlighted) might make things clearer.

It is not usually Immigration who questions this but airlines. However, that was my experience pre-pandemic 0 it may have changed.