On my last trip to Myanmar in 2016, I entered by foot at Mae Sot and left by boat from Kawthoung to Ranong Thailand, this is the most southern crossing and walking distance to the main road from Krabi.
So entering and leaving at different crossings was no problem.
It was possible to enter in Kawthoung. I didn’t have an eVisa so I don’t know about that.
The road up north from Kawthoung was open for walking and hitch hiking, but there were also buses.
I don’t have up to date information but I don’t expect the rules in this part of the country have changed.
Credit:stackoverflow.com‘