Can I extend my 45-day visa exemption by an additional 30 days in advance?

Yes you can apply now and yes it will be added to your existing date.

Yes, you can. No problem

You can apply for extension when you have 30 days or less left on your existing visa exempt at most offices. So in another week or so.

In Bangkok, Phuket and Chiang Mai you can apply when you have 45 days or less left, so you could apply now if you're in one of those places.

Chonburi/Jomtien and Samui only let you apply during your last week.

Regardless of when you apply, the 30 days will be added to the end of your stay and you won't lose any days. Also it's almost unheard of to be denied so you can book your flight just assuming you'll have your 30 extra days.