where I assume away any compensation for the initial dip...Garber's data are also consistent with an initial rebound.

No, you don't. You bury it in the 'sank gradually' part:

But Garber would likely have seen the same thing even if there had been an abrupt bubble pop. Suppose a tulip bulb's price peaked at 1000 guilders, crashed to 200 guilders within a week, then sank gradually to 100 guilders over the next five years.

You can get an abrupt pop inside an normal-looking beginning/end comparison if something compensates for the pop, like another rise (unlikely) or prices then falling slower than they normally would ('gradually'). The ground lost in the pop is then made up later.

Plague? Now that's something I don't think he mentions in the papers.

His book's capsule summary of that bit goes

The speculation in common bulbs was a phenomenon lasting one month in the dreary Dutch winter of 1637. A drinking phenomenon held in the taverns, it occurred in the midst of a massive outbreak of bubonic plague and had no real consequence.

It's the topic of chapter 5, "The Bubonic Plague".

(Must...resist...urge to borrow...yet another...book.)

(It's on Libgen, and isn't a very long book.)