Should be fine. You crossed back into Thailand with a tourist visa so that wont count as a VOA/VOE. Someone must just correct me if i am wrong but i think you will have 2 land border crossings available to you. There is no limitation when you fly. So always keep your eyes open for cheap flights.

Have you translated the writing? It might just say prohibited to work, so, if that's the case, then they haven't flagged you. Your old visa's never fall away, they just lose their validity. Depends on the immigration officer on the day if he/she will allow you to cross back into Thailand. You will need 20 000 Baht cash, proof of accommodation and onwards travel outside of the kingdom.-For a tourist visa.

If you plan to stay less than 60 days, just do a quick border run and then an extension, otherwise if you plan to stay longer do a tourist Visa.

With regards to costs of Cambodia, I presume the costs would be very similar to a trip to Laos, but wait to hear from someone who has gone that route.

Best of luck

Smart analogy.