This might seem like a really dumb question, but i've never been to Eroica. My apologies in advance. From my understanding, they're big about only allowing "period-correct" bicycles ride. Would such a modification violate the rules/spirit of the event?

Some possible solutions for your predicaments:

Have you checked the chainline of the crankset with your modification? Maybe your setup is correct, and it's the chainline that's wrong.

An "alpine" front derailleur would probably solve your slack chain issue.

When your chain goes slack and hits the cage on the highest two cogs in the granny, you're probably exceeding the chain wrap capacity of the rear derailleur. Do you have a suntour mountech, or a campagnolo RD with soma's new cage plates?