Is there any way at all to add the works to my trip after booking, or is that a one-time buy it now or kick yourself later situation? Not that I've noticed. I wish they would have an option to add this later on but I don't believe its possible at this time.

Is my only option now to suck it up and pay the stretch fee and baggage fee ala carte? Yes, unfortunately.

What is Frontier's policy on discounting last minute? If the plane is still rather empty the last couple of weeks before the flight, is there a chance it will go back down to $45 again? There is definitely a good chance that it will be discounted again, although its anybody's guess as to how much the discount is.

And if I chose to put a nice carry on bag in that second seat instead of someone else's rump, would that be kosher with Frontier, or would I still have to pay $30 for a carry on? Definitely not kosher, a seat is different than a carry-on, and in many cases, a carry-on costs more than a seat. You would still need to pay if you wanted a carry-on.

If I bought two seats in my name and didn't pay for an assignment, but checked in at the same time for both of them, would me and myself be seated next to each other, or would I have to pay for a seating assignment for both of those seats? Because of the way Frontier's random seating algo is set up, there's a good chance they'd be apart, but a GA would probably be able to put them together for you at the airport.