, your departure flight would have to be before the end of your arrival conditions, which in this case would be the 45 days. (Doesn't matter what you plan to do after you actually arrive.)
As you can see, everyone has different experiences. I have been asked a couple times by the airline, and a couple times by Thai immigration. So, always best to have one, even if you cancel it later. Also, on a related note, have some cash on you when you arrive, in case you are asked.
You don't need a visa if you are visa exempt, as you will leave the country after a week or 2, then only be in country the 2nd time for less than the 45 days again. (I believe the 45 day exemption ends on March 31st. If not, you can still get the 30 day extension.)
You should have one. As they say, immigration probably won't ask you. But IF they do ask, you had better be prepared to produce one. Saying that, I think the airlines will ask more than the Thai immigration.
I flew in a few days ago, and they asked the guy in the line next to me at immigration.