As long as the Immigration officer, after reviewing the posters stay history, accepts that she continues to be a tourist. Otherwise he will deny her entry.
If this is your third visit within a 12 month period, the Immigration officer could easily believe that you are no longer a tourist and could deny you entry unless you have a visa valid for the reason of your stay. Be warned.
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.