Agents: as others have said, agent numbers grow (along with armies/fleets) as one meets power milestones along the way ("Imperium" for Rome, probably called something else for other factions).

Marriages: I haven't figured out anyway to get one's own politically-significant family members married.

From an immersion standpoint, I agree, this is kinda stupid.

From gameplay standpoint, some comments:

First, it's another slot alongside traits, household, skills. The wives do have effects, generally affecting gravitas as far as I've seen. So, yeah, not being able to arrange marriages for one's own characters is annoying in this regard as well.

Second, worth noting that marriages seem most desirable when one's family is the underdog, as the less powerful faction gains senate influence while the more powerful loses. Interestingly, only when my own family was underdog toward beginning of the game did I get a couple of marriage proposals from other families. Once I passed them up later and became top dog, haven't received a marriage proposal since. This seems counterintuitive from the AI's perspective. I'd expect that because I now control about 55% in the Senate, the other families would be falling all over themselves to get their daughters and nieces into my family in order to counteract this trend.

Final comment/question. I received a (possibly erroneous) impression from reading pre-release reviews that if one's senate influence gets high enough, then a civil war is triggered, as the other families band together against the dominant family. For this reason, I've tried not to let my Senate influence go above roughly 55% or so. When it goes higher, I marry off some nieces to whichever other family is lowest. Should I bother? Are there benefits to gaining total senate control which are worth the headache of civil war? Is there in fact a discrete "civil war trigger", or is it totally random? Or inevitable, and will eventually occur regardless of what I do?

For that matter, is the political power "mini-game" even worth much attention at all? Obviously individual generals' ranks are important, as they gain skills. But is a family's overall Senate standing significant? My Junii at one lowpoint (lol, several defeats while trying to figure out naval warfare) got down to about 9%, but have been as high as 62%. Haven't really discerned any noticeable penalties/rewards in either direction. The political rank perks (praetor, aedile, etc) are nice, but I don't necessarily see that they're worth spending a lot of time/attention (not to mention treasury cost) to achieve them.