How to composite musician into this footage?

By inverting I simply mean you add or subtract whatever values the track generates from your other motion. So let's assume your keyed work is static smack in the middle of the comp you would copy & paste the tracker keyframes onto it and use the formula I already provided to invert the direction and potentially tweak other aspects to make it look "real".

Mylenium

The only thing you have to track in your shot is the pan itself so you can invert it and apply it to your keyed footage. A simple position track of some points on the distant shore line will do. Then you can use a simple value*-1*multiplier+offset style expression to derive any counter motion to create the parallax. For the rotoscoping you'd focus on creating a primary clean plate where you remove the water and trees. From that pre-composition you then generate the actual layering by applying additional masking to separate the steering wheel, the girl and whatever else you need plus you'd use the initial roto as an inverted matte later to re-introduce the parts of the original shot.

Mylenium

That shot has very little parallax shift because it is almost a simple pan. The background is far enough away that you could motion-track the background using Mocha AE. I would probably motion stabilize the shot because that would make creating the track mattes easier. Then drop in your keyed footage, and if necessary, add a little movement on the X-axis to simulate a slight parallax shift. When that is done, you add a null to the timeline, apply the tracking information to the null, and then parent all of the pieces of the composite to the null to put the movement back in the shot.

Personally, I think the motion stabilize, composite, and put the motion back workflow is going to be the least amount of work because the required rotoscoping will be easier. You could just apply the tracking to the keyed musician, use a procedural matte to knock out the car frame, and roto the girl that walks through the shot. 

I hope this helps. Here's a link to Mocha Essentials.  The training series should give you just about everything you need to track the shot and prepare for the composit.

Seems simple enough, but the shot has a very distinct curvature. Chosing a correct lense and figuring out a good distance to the talent will be key. Other than that there's of course a lot of softness on the exterior, so integrating your artist won't necessarily be straightforward. There's a good chance you will have to draw some complimentary feathered masks and animate them by hand.

Mylenium