There are two ways to access the domestic area in BKK. 1) go our through immigration, go up to the fourth level and then go through domestic security. 2) Go through immigration at the junction of A,B,C,D concourses to access the domestic concourses. I would suspect #2 also has security involved.

You complain about not wanting to go through immigration because you're going to Samui. Well, Samui is in Thailand. You're trying to take a domestic flight.

And I don't believe 70 min is a legal connection, but, even if it is, you're taking quite a risk doing this with 2 PNRs.