DO NOT BEG. It will ruin your bargaining strength in the future, when you “need” to buy the next one. Yes, if you stick with this sport there will be a next one, but by then your wife may see you’re getting your money’s worth.
Depending on the tire, a good Muni may be not-so-great on paved streets. People in this thread can suggest tires that are good for both. If you aren’t planning to ride on lots of mud or snow, you don’t need an aggressive tire.
I’m going to go with the general consensus here and say 26 or 27.5. Don’t agonize over which of those; it’s not that big of a size difference. One is slightly faster, one is slightly better able to handle the technical stuff. My first new Muni since 2003 was a 26" and I love it. My previous was a 24", but with 3" tires, your actual tire diameter is bigger; probably about 26 on my old 24, and 28 on my current 26. Don’t agonize, just pick one of those. For cranks, I agree that a 2-hole system will offer you an easy way to try multiple sizes. Do the 125/150s. 150 for dirt (at least in the beginning) and 127 for street. 127 will also be fun on dirt if the trail isn’t steep or technical. Get a pedal wrench.
Short answer: It just does; bikes have been doing it that way for at least 130 years. For more about the why, consider the word “precession”. The force that the crank feels is not what you do when you pedal, it’s a spiral of force, around and around the threads.
That usually applies to a tire with aggressive treads, or a squarish profile, when you ride it on a slope, or along the edge of a cambered street. Some tires are better at letting you just ride straight, while others tend to want to aim you downhill. When riding perpindicular to steep slopes, some tires may also want to steer you into the slope, though that’s the exception (and not necessarily a bad thing…).
It’s easy enough to use clear logic on the second uni purchase. The learner is great for the basics, and can be used for the next person who wants to learn. But it’s slow and not good on bumps. Not as good of an exercise machine. It gets harder to justify the additional wheels as you start filling in all the major sizes…
(I have, or have had, multiple unicycles in most of these sizes: 12", 20", 24", 26", 27", 36", 40", 45". However most of these were acquired prior to me being married…and she married me anyway! :))