I would also explore the stated need to allow company employees to use their corporate login. How critical is this to your solution? Could you launch without the integration? Have customers asked for the integration?
There are plenty of b2b SaaS solutions that don't offer this type of integration for variety of reasons. As a startup, you don't have the luxury to go down this rabbit hole if your solution is not dependent on it. Focus on your core solution and integration can be added later.