Overland Thailand to BurmaCountry-Thailand to Burma
all allow shore trips mainly (there have been some detailled places on Trip Advisor re entry/exit via shore at the south frontier points)...the only one that definitely requires a landing trip is the most northern transition at Mae Sai to Taichileik.
As mentioned above, you can only get there by road to Kengtung, by coach or vans, then you have to go by plane (to Heho/Mandalay/Lashio/Yangon). From where you start, the frontier is the'fastest' for you...e.g. if you start from Chiang Mai or Chiang Rai in Thailand, then it would be fastest to drive via Mae Sai-Taichileik...you could drive to Inle on the same date (by coach to Mae Sai, across the frontier, arrival flights from Taichileik to Heho & taxis to Inle).
When you are planning to rent a vehicle and a rider to see "the attractions of Myanmar", then obviously you are not worried about $$, and if you are only interested in Bagan/Inle, then I don't know why you think of a country boundary at all....? Renting a vehicle and rider from Myawaddy/Mawlamyine Tour to Bagan/Inle would be more expensive than traveling from Bgk-Mandalay.
You do not need a permit....as long as you have your Myanmar visa...then you are flying from Taichileik to Heho. 2- there is no VOA....and since its inception, e-visa has only ever been for aerial entries...unless that has been changing lately...but I have not seen any postings here or TA indicating that they are now fine for entry to national borders.....?
No, there is no'additional fee' at the country boundaries..... Actually, there are only 4 sets of transitions internationally. Further crossing points could be opened in the near term, as various newspapers in Thailand and Myanmar suggest, but so far all other crossing points are only for Thailand tourists, who are only permitted to go on daily excursions near the limit.
A peculiar exemption is the one at Dan Singkorn intersection - Thais are permitted to drive to Myeik from there, but not elsewhere. On the other hand, people from Burma can only go to Prachuab, 12 km away and for no more than 3 nights, while Thais can stay in Myeik for up to 14 of them.