First, it sounds like you are a better spinner than me, and I don't think I've ever had Mark IV's, so take this with a grain of salt. But spinning can be hard to learn late in life, even if you done it before.
If you practice once or twice / week or more, I think June -> December is more than long enough to get used to your skates and blades.
Fitters are told to select the blade length and mount position by the boot outsole length and position, but there is usually no reason why that should happen to be best for you, and make no sense whatsoever. But the the overall length of the boot is completely irrelevant, because the front of the boot does not touch the ice. A really, really long boot would be a problem in the sense you couldn't bring your feet close together (though it would still look like you could), but I think you would know if that was an issue.
Even a good fitter can guess wrong for your particular body. You need a boot that hugs your heel, has the bend in the foot bed where the ball of your foot is, with an amount of bend that is both comfortable and lets you feel in control of what your skate is doing. As much as possible, the boot should be snugly comfortable all over.
As far as blade shape and length, where the blade sweet spot is relative to the ball of your foot matters, and how easy it is to reach your toe pick. If the sweet spot is just a little bit forwards of the ball of your foot (where it bends), which makes it easy for you to control motions back and forth across the sweet spot, and the toe pick feels like it is where you want it to be, then the blade is right for your spins. And the amount of heel should feel comfortable and natural. Those things you can check. But you have to be able to feel where your sweet spot is to take full advantage of it - so if the other things check out, ask your sharpener to work on that. The blade length therefore the position of the back of the blade matters if you lose balance, or if you tend to accidentally cross step one blade on the other - but that's not a spin thing.
A good sharpener can emphasize the sweet spot (make it a sharper transition or make it change directions a little), to make it easier for you to find. He/she can also move the entire blade forward and back to play with the toe pick position. If spins are all you are worried about, you may not need to change blades.
Of course, the Mark IV will wear out faster than higher level blades, because the steel isn't as durable. But that's in the future.