In Czechia you need documents that are translated and cross-signed by Ministry of Foreign Affairs and Ministry of Interior, and local Thai embassy could not even help with where they exactly sign papers like these... I gave up, locally. Documents itself are simple, but signatures / translations are not. Much easier to do in Thailand :)
yes, very clear, you can enter 5 times for 180 days, and each stay can be extended for 180 days, aka you can stay for full 5 years (just have to leave for 1 hour each year and come back).
as I understand it, this is not neccessary if original visa is still valid when I return. It would be neccessary only if I would return after visa expiration day.