I do believe you can achieve the look shown in the last image, but it won't be easy. The hardest parts with any HP is the front hairline and the sides, especially with a high fade. In my opinion, with your proposed style, the front will be easier to master, especially if you pull a few strands of hair down to break up the edge of the hairline. The same would be true of the sides. Both are do-able, but will require some work to figure out the details, so don't become frustrated. A casual (sometimes called messy) hairstyle will definitely make it easier to achieve. Do it in stages, rather than taking a drastic trimming of the HP since you can't put back what you cut off.

Even now, with years of experience wearing HPs, it sometimes drives me crazy to figure out how to achieve a natural look (as revealed in the above pic). In that case, with no hair product on the hair, it just did not work. Simple solution: use a pomade or gel.

One other thing: I would not shave your remaining hair (underneath the HP) until you figure out the optimal parameters for your new HP.

Take home advice: just do it, but cautiously and with patience.