Can I renew my Non-O retirement visa just before its expiration on April 17th if I return to Thailand on April 15th?

Well, it's either gonna work out flawlessly or the O/P is gonna come back into the country and then run right off the rails running it so close to their expiration before applying.

I'd say IF you use an office that allows 45 days or less, IF you meet the financial requirements and can go before you fly out (March 3rd or later), that'd be the way to do it. Get your new year extension AND a re-entry permit before you leave.

Take your proof of flight out too (just in case you're using an office that doesn't do it 45 days or less, so you can show you will be out of the country. I have had that work in getting extensions issued earlier than normal)

Best option is to extend it before your leave

April 15 is the Songkran holiday and April 17 is on the weekend.

Your only choice could be trying before you leave. Go to your local immigration office, bring all documents and FIXED roundtrip air tickets and talk about the possibilities of you can apply earlier even your local office has 30days-in-advance-policy. The worst case you will be stamped until 17th April, prepare every documents and application has to be done on 18th April Monday (<-, only when you would be stamped for 1-day overstay, but overstay fine would be waved due to the office close and can apply for an extension as usual)

Most Immigration Offices let you apply between 45 days before and the last day. I would say go now and get a re-entry permit once the new Extension is stamped in your passport.

If you come back on the 15th, 1 day hotel, you have a little time left to organize all.