Am I restricted to the return date I wrote on my Thailand re-entry permit application?

Best to put the earlier date, but mai pen rai.

You're under no obligation to the originally supplied intended date of return

Make sure when you return you help the immigration officer by double checking your stamp as soon as you step away from the entry desk. It's becoming far too common to get an incorrect stamp on entry, especially with the pressure on the immigration officers to increase throughput. It's very easy to fix if you catch a mistake there, and much more complicated if you discover it later.

Nope, not at all, you just have to put in some date on the re-entry application form when you apply for it

BUT

Your re-entry permit is valid for the same date your current stamp was valid for so you can come back when ever you want (as long as it's before the date your re-entry runs out 😉 )