How long are these valid for? if I came back to Thailand and it was a few weeks over the date stated on the permit would this affect my extension to my non imm o visa? Or can you even leave the return date blank?.

A re-entry permit is valid for the duration of your current extension. On the Re-entry permit that is a valid until date this is the same date as your extension, it cannot be used after this as it goes hand in hand with the extension/Visa.

The same validity as your current permision to stay.

The same validity as your current permision to stay.

thank you, I seemed to remember the form asking for dates of travel, but i guess this is just a formality and it's valid from date if issue until extension expires.

Yes. It does not really matter what you put on the form.