Pretty easy actually. Assuming Krom's other options are already married/dead/never got within a foot of him, 3 dances from Olivia in Chapter 11 are enough. Really, it's keeping him single until then that's the problem--even if he doesn't partake in conversations, just being capable of a C support can be enough for someone to outbid Olivia.
As for conversations, he doesn't really have them with Olivia or the Villager the way he does with the others. Chapter 11 just ends without making Krom's bride clear, and then the time-skip. At the start of Chapter 12, Krom's wife shows up w/ child, and Liz jokes she was surprised how fast Krom works, teasing about how he'd gotten into a relationship Olivia/found a bride (Villager) before anyone could notice. In both cases, he responds with "Don't tease me, Liz. A lot happened in the past few years," followed by a "flashback" to Olivia's S support in her case, and nothing for the Villager. And then plot. No real marriage conversation to speak of.