You may also need the certificate legalised by your home country before it can be certified by the Thai authorities. This is certainly the case for UK marriage certificates, but possibly not Taiwanese.
As stated above, 90 day reporting is still required and you get permitted to stay for one year. There was a thread here a few weeks ago from an Elite visa holder who overstayed as he thought he was stamped in for 5 years!
Actually 90 day reporting is required for Elite, although they will do them for you if necessary. It is also necessary to leave/re-enter once per year, as your passport is stamped each year, during the 5 year visa period.
That's what I thought. Flying from BKK is much more convenient than flying from Laos, so some expats who fly regularly must need to cross the border a lot to do so. The METV takes care of that.