The rules you've quoted on combining magical effects are unambiguous - these two effects stack, since they're not from the same source.

As for how they stack, the answer is your fourth choice. While in previous editions the rule was that multipliers stacked additively, not multiplicatively, 5e has no such rule. Likewise, 5e has no concept of "base value". When you use Step of the Wind, your jump distance is doubled. Then when you use Jump, your jump distance is tripled.

It should be noted that High Jump, like Long Jump, states that each foot you jump costs a foot of movement, so the distance you can jump is still constrained by your move speed. Obviously there are numerous methods of getting a very high move speed, but it is still an important constraint on your ability to leap skyscrapers in a single bound.

Also worth noting is that, as the highest rated answer on your linked question says, you still take falling damage when you jump, so another constraint on your ability to jump into the stratosphere is the damage you will take when you land.

And finally, to dismiss any concerns about balance, I have to point out that this requires a 1st-level spell and a 2nd level Monk feature to pull off, and Fly is a 3rd-level spell. And that's not even mentioning the fact that you can reasonably expect to be fighting flying enemies right from the start at level 1. This "super jump" is a cool trick, but ultimately not that useful or powerful.

The spell with the most images would become active (all else being equal)

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap.

This is going to come down to how the DM rules how to measure potency as the game doesn't tell us a definitive answer for what should count. It seems reasonable to say that, all other things being equal, that the spell with the most images would be considered the most potent effect at the time.

How this would play out in practice would be that whenever a spell's number of images is reduced below that of the other spell, the other spell would become active. When both spells have equal images (and everything else) it really doesn't matter which spell is active done they are identical. The alternating behavior would continue until one spell loses all of its images.

What if all else is not equal?

The above assumes that the spells are identical in every respect (duration, AC, etc.) if there is any other discrepancies the DM will have to choose how they define potency in more detail. For example, if one mirror image was cast a round later than the other one it would be completely reasonable to rule that the one with the longer duration remaining would be the more potent spell and that it would override the spell for the entire duration. This would also be the case if one of the spells was cast at a higher level than the other (and everything else was the same).