Do I need to obtain a visa before arriving in Laos, and will I face issues re-entering Thailand after canceling my ED visa?

You can get the Laos visa on arrival at the border or the airport on arrival in Laos. Your re-entry to Thailand is solely down to the Immigration officer.

A letter stating cancellation of your ED visa means nothing. The cancelled extension in your passport is the only that matters. You have to use that letter to cancel your ED extension before you leave.

But regardless, you're likely to face some scrutiny for having any ED visa in your passport after all the investigations after the past few months. Land border with Laos is known as one of the easier ones to cross back into Thailand though. Just make sure you have the 3 proofs so they cannot easily deny you entry.

1) Proof of funds (20,000 baht or equivalent in another major currency IN CASH ONLY)

2) Proof of leaving Thailand within the number of days you'll be stamped in for. (A ticket out of Thailand within 30 or 45 days)

3) Proof of lodging (Hotel reservation for at least the first few days of your stay)