2) it ends on 27 September, not 30th. As to when, if, they announce it is being extended or not...anyone's guess! Last time they announced a week or so in advance, previous time they announced it on the afternoon of the day it was ending.
Check your local Thai Embassy website about 2-3 weeks before your departure. Things change daily here so what's required today might not be the case in 4-6 weeks time.
No, you can still travel there. Presumably you also would've done mandatory quarantine to enter Thailand. However, your application is unlikely to be refused anyway provided you provide them with whatever documentation they require.
not everyone is a national of those bordering countries so as far as the Thai govt would be concerned you can fly to the country of your passport. There have been reports of some immigration offices asking for proof you can't go back home in order to provide the unable to travel due to covid extensions.