I don't know if this is accurate (so why am I saying it, LOL?), but I picture the late nadir thing like when the insulin is actually at peak (so say +6), their body recognizes it is too strong so resists it, and then once they are past that point their body says "ok, NOW I can use this insulin w/o being in danger" and then you see the better #s late in the cycle, once the "danger zone" is past. I realize this is totally made up in my head, I really don't understand what is *actually* going on, but the mental picture helps me make sense of it in a way that doesn't drive me screaming from the room. Then if you ease off the dose a hair and get a nadir at like +6 of 55 or something, and a more normal curve, it confirms that kind of thinking (for better or worse, LOL).
As for the jump from a shot at 138 to a +3 of 300, PZI is known for its dramatic "poop out", so if you are getting good duration like you are, you can have good #s up til whenever, +13 or so, and then when it's done it's done, and you can see mega-zoom after that. With the lowered dose it will be more dramatic than on a higher dose, and also if you shot later than +12 you'll probably see more drama zoom before the next shot kicks in.
Evaluating the data to some extent depends on when you gave the shots - sorry I am too brain dead to get my mind around the details right now of when you actually shot. But basically if you shot the 1.4 at say +13, you can figure it will have a slightly stronger effect when shooting at a regular +12. Often normal-ish doses shot at +13 or +14 will at best keep the #s flatish. It's hard to get a drop down to a good nadir on a late shot, you can lose overlap pretty fast, and then the insulin has to work harder. Usually on a late shot if you want to hit a good nadir you would raise the dose by maybe 0.2 or so, or if you just want to maintain the #s til the next shot and then get back on schedule, you would shoot your regular dose and settle for a flattish cycle.
So off the top of my head I'm not sure what I'd recommend for your "regular" dose right now. 1.4 definitely looked too low, but then if it was a late shot it might not be so bad on a +12. 1.8 looks like you may be getting more low PSs, but see how tomorrow looks. I would view your current situation as data gathering - try some different things ranging between 1.4 and 1.8 and see what results you get. If PSs creep higher raise a bit, if they creep lower, lower the dose a bit, and of course looking at nadirs too, at least when you get them at more normal times. Late nadirs I would kind-of ignore in terms of the #s themselves, and just ease off the dose a hair and see if that gives you a better U-curve.