If you can no longer edit a message, it means that the person has opened it?

yes, but it also can't be edited after a certain time frame

I think there's an expiry limit to change it too?

Or more than 15 minutes has passed

Yes if you can't edit it they opened it or the time has elapsed, I think they give you 15 minutes then you can't edit it even if they have not opened it.

That and a time limit after sent

Too late you should not have told them about the diaper fetish if you did not want them to know.

yes

you also can't edit it if 15 minutes have passed since you sent it

Yes (si)