I think a good part of Boeing's reason for building the 747-8i and -F is that they simply refused concede that business to the other major airframer. The freighters are starting to pick up and, as you note, a bunch of customers who can stall delivery as waiting for the improved spec engine. With four burning and such heavy loads, even 1% or 2% fuel reduction makes a huge difference in operating costs. I don't know, but I'd suspect that it is not possible to retro fit the PIP into existing engines.
As for the -8i PAX model, my guess it was just a convenient add-on to the -F program, with 95% of the engineering already done. The capacity differences between the stretched 777s and the A380 are just not all that great. Othe than a handfull of VIP airplanes, I think LH is the only 8i customer on the books. Ha-ha, maybe Boeing is just holding out for the Air Force One order - as if they don't already have it in the bag! Can't you just see the U.S. President noodling around the world on a French made jet? That just won't happen, so the modified 747-8i is the only game in town that meets the basic specs, one of which is four engines. The 747-8 series will sell in time...