I would like Spike to appear at some point, should bring the Lena family drama to a culmination, whether she finally reconciles, working their differences or finally break, leaving them to their probably debt-ridden fate. But other than that he should be a one note character. Would be funny if he was one of Emma's crazy roommates though.
On the Mike-Gillian theory thing, I think Ian never knew who the guy was. If I understood correctly, according to Mike they probably met in their hometown where Gillian was eventually gonna take her Master's and hooked up there. Mike only arrives in Baluart 3 months before the start of the story, if memory serves.
But I'm 100% sure Jeremy knows, he always acts a little off when Mike's girlfriend is mentioned.