Answer to first puzzle:- when the 'split' PRU scheme was adopted (i.e. grey/PRU blue) the diameter of the fuselage roundels was enlarged, so that they didn't merge, or 'fuzz', at a distance, which would have been the optical illusion caused by the 'split' of two similar toned, but different colours, along the top quarter of the roundel.
I've given Airfix some 'stick' on this forum recently, but, judging from what I've seen of this kit, I'm guessuing the research will have been fairly thorough, so I think the decals provided are probably correct. Bear in mind, of course, I haven't seen the kit or the decals first hand.
Answer to the question re printing of decals; I might be wrong, but I would think it's a question of economy and accuracy. That is, printing plates can be produced to print the decal as a 'one piece', but the process, depending whether digital or silver imaging (photography), would demand much closer tolerances to acheive the exact register of the separate images for each colour, in order to produce an acceptable product for 'todays' market, and the production costs and time that go with this. For the roundel alone, it would require a minimum of 4 plates, possibly eight, in order to obtain the required colour tone, density and register.
As many decals are now produced by silk-screen printing, or by digital direct printing, it is much easier, and therefore more cost effective, to produce such items as the red centre-spot as a separate image on the original work. It's really a question of who does the job, and how. They can still be produced as in the 'traditional' way, but might cost a few pence more per sheet (hundreds or thousands of $/£ etc per print run), which could affect the base price of the finished product (the kit) to the point where the decal sheet (normally produced by outside sources) could cost, per unit, almost as much as the production cost of the kit itself.
Personally, I find the separate centre-spot a bit of a bind - but - they do allow a more dense background to the white part of the roundel, where a complete decal might, due to the printing process, provide a 'thin' white ground, where the base colour of paint beneath, particularly any demarcation lines, as in camouflage colours, would show through.
Regreattably, it's a 'sign of the times'. Back in the year dot, when all decals were glossy and thick, the production wasn't a problem. But now, due to a combination of changes in 'traditional' printing techniques, and market acceptance and expectations, decals are, generally, produced thin (to conform), and accurate (as in colours etc), to match the aftermarket products at least.
I wonder how many of 'todays' modellers would have any experience of drawing, colouring, and then making, their own 'transfers', on gummed lable paper?!