I wish I had more answers to your specific questions. But there are plenty of qualified people here who will.
My guess is that the departure tax is not included in her ticket if she is traveling on a single ticket. Pretty sure it is still 700 baht. Last time I had to pay it was cash only.
There are so many tour guides/companies that I am sure you should be able to find some that will be happy to meet her at the airport, particularly with it being low season there should be plenty of folks there who would be happy to work with you. Unfortunately my go-to guide is in Paris now so he is no help in this situation. On a Saturday AM that early there shouldn't be too much traffic. I would just sign up for a personal tour with car and driver. Of course that also depends somewhat on how much you are will to spend and what exactly she wants to see or do. Arriving that early in the morning though not a whole lot of attractions will be open yet. But I believe the Grand Palace opens at 8:30 so she wouldn't have to wait too long. Or she could go to the flower market to start there.
If she is not planning to stay at a hotel why does she want to go to the Aloft? It's a decent new hotel, but the soi is nothing special. Not worth going out of your way to visit.
Taxis are always around. I would NOT take a tuk tuk unless my life depended on it. And even then I'm not so sure. :-)