@Tee ********
Great, therefore you've done the right thing. Printing the e-visa is to be expected, because they tell you to do that in your approval email. On the other hand, I'm surprised they're asking for proof of having done the DTAC thing. Some passengers will only do it on arrival and I'm pretty sure those machines in the arrivals concourse don't spit out a paper receipt (of course, if anyone can correct me there, please provide evidence).
From my own experience - having done the DTAC twice now since it's inception on May 1, I re-entered Thailand through a large border for my first post DTAC re-entry. No proof was requested and I didn't have any proof of having done it (well, technically, I could have shown the email I received had that been asked for, but it wasn't).
Second re-entry through a smaller land border and immigration couldn't find it, so they asked to see proof. I went back to my car to retrieve my phone and showed a partial picture on my phone and they finally found it.
Therefore, Suvarnabhumi airport of all places should be the most advanced and least likely to require a paper print out, or even a QR code...everything should be in their system. It seems like smaller border crossings may require some sort of proof, while larger ones will have the capability of seeing it in their system without needing to show proof.