I agree with ra.ra that there was quite a bit of either continuing experimentation or parts substitutions going on at Advent (same with Bose). I've seen paralleled caps as mentioned, and paralleled resistors to bring some of the old 4Ω sandcast resistors down to 3Ω. That bluish Nichicon is a toss up, though, and hard to rule out it being a later mod, for the reasons you said.
There is a great similarity between the Smaller Advent crossover and that in the OLA when the latter is switched into the "increase" or "extended" configuration. Using the photos above, the hot lead from the outside comes in on the lower left, with the red wire going off to the woofer (presumably through an inductor first, probably mounted somewhere else on the cabinet). The tweeter leg starts at the same hot lead, first through the 16µF cap and then through a 8µF cap in parallel with the resistor (3Ω in the OLA). From there, the tweeter leg continues to the hot side of the tweeter via a yellow wire, and the black wire is the return (for both the woofer and tweeter). The button inductor on the right is in a shunt configuration, straddling the hot (yellow) and neutral (black) leads to the tweeter.
I think going with 12s and 8s is a reasonable choice. Once you get things working, you can always consider tacking in another 4µF in parallel with the 12s if you are curious.
After recapping a recently acquired older pair of OLAs that had the same black caps with the red ends on them, and listening to them, I was reminded of the sonic quality that Henry Kloss was able to deliver to the market for not very much money. The bass is deep and tight and defined, and the high end, while not bright, is well dispersed and transitions so smoothly from the woofers (then again, I am old enough that I don't think I hear much beyond 10KHz). I think you will like these once you get them fixed up.
Well here is what I found in the first box!
