You should have some sort of disaster preparedness plan in place anyway. There is such a thing as key person insurance but I think that's designed to address permanent loss of a team member and I have no idea about the economics. It probably wouldn't cover this situation but it seems like something to be aware of anyway.
It's nice that you are taking this issue seriously rather than waiting until the law says you have to do so.
The moral of the story is that you should accommodate your team member in any way possible or this critical person will leave after their maternity leave for good. If they are so critical to your team that you feel you cannot lose them for a month (about 1/3 of what you should give them), then can you really afford to lose them shortly after they come back?
As for how to handle this in terms of salary: you should offer short term disability insurance to your employees that covers maternity leave. It's too late now, since you typically have to hold the policy for over a year before maternity leave is covered, but do this for the future. Good luck!
First, figure out exactly what makes this person so critical - it may seem obvious, but break it down into specific tasks and skills. Maybe get them to write down the things they do for each of these that took the most time to figure out. Second, evaluate who else on your team can pick up a specific skill or task, or who would best be able to grow into that particular role element (rollette?). Finally, spend some time and sweat encouraging the rest of the team to grow into these areas - make sure to show them you appreciate their taking on the challenge. If no-one can step up for a specific rollette, you may need to start recruiting.
Kudos, by the way, (as anigbrowl pointed out) for wanting to do the morally right thing. Good luck
As for whether you should offer mat leave even if you don't have to - absolutely, it will leave a bad taste in your teams mouth if you don't, and will probably make anyone considering having children resign
The founders should be willing to take on the extra critical work if her minimal contribution during the period isn't enough. If this takes more than a small amount of their time or she will not be able to do anything (hospitalization, complications), then you should be hiring another person or a contractor for the role anyway.
Have you talked with her to get a sense of what she's thinking and what an ideal future work scenario might look like? Be sure to communicate her importance to the team.
Cover the salary on-leave, it buys tremendous goodwill. But be fully prepared in case she doesn't return from maternity leave. If you haven't been there yet, it's a life changer on a variety of levels.
If retention becomes an issue-- consider allowing her to work virtual office and/or shift to part-time for the next several months.
Don't make the paid leave contingent on her returning, or you'll have negative feelings about her participation and possibly about her peers on the team. Mentally, call it a severance package, even if she intends to return. Let her and the money go, and focus on overall contingency plans, as mentioned by others.