1. Yes, I suggest that you apply before arrival. 2. Once you are here and have your Retirement visa, then you can go to Immgration and purchase a single or a multi re entry. 3. There is no m inimum stay requirement. 4. It is straight forward and simple to do yourself - why pay an agent extra money.
I am confident that the Thai police know and understand the laws relating to licences from ASEAN countries which are totally different to the laws for licences from Western countries.
There are no valid international drivers licences although you can obtain an illegal one from the internet. Perhaps you mean an Intrnational Drivers Permit (IDP) but then you would still be wrong. The poster has a valid licence from the Philippines which is part of ASIAN and as such is valid in all ASIAN countries including Thailand and does not need to be accompanied by an IDP.
To renew for a second/subsequent year, Immigration may want to test that your Thai has improved to an acceptable standard. Get a one year Thai education visa and then try a different type of educaction visa.
There is a wonderful Thai lady ojn the staff at Bangkok Bank in Doi Saket. She has willingly assisted many Westerners to obtain a bank account. You will need yopur passport, letter of residence, deposit of 100,000 thb and it will assist if you are accompanied by an account holder of the branch.
Best to get your retirement visa before you arrive. If you arrive without a long term visa, it will be almost impossible to open a Thai bank account to deposit your 800,000 thb which is a requirement to obtin the visa in counrty.