How are current money value and enemy level linked? I noticed when I hoard large sums, enemy level suddenly skyrockets. 

For funsies, I sunk almost all cash generated over ~150 in-game days into 3 goldmines, all with "mineral rich" bonus on their tiles; they are all lvl 15, at least generate upwards of 10.000/120s. After hoarding ~750000  coins, I got a lvl ~440 NPC in the Tempt Invader tab. After hoarding over 1 mil, I got lvl 1040! Then I did another upgrade round, had only 10000 or so left when market re-stocked, next invader was lvl 23. 

Also noticed the first invader in the Tempt tab on the altar and the first (left in the row) slave in the market will usually be much higher than the rest -if your money is extremely high- or much lower level than the rest when your money tends towards 0. Thought the money score just influenced invasion probability. What gives?!?

Can anyone explain to me how this is all connected?

For what i can understand from how money works in the game is that it entice stronger enemies to invade your dungeon, and that it also gives better items to buy at the market.

For the probability of getting raided seems to be more about how much time has passed without getting raided. So it might be because that since you always get raided or tempt invaders, it lowers its chance of being invaded.