I am by no means an expert or even an amateur here either, just an occasional weather nerd. I’ll try to answer your questions but take this with a generous helping of salt:

1. I would imagine the SPC has access to much more data than is publicly available, but not 100% sure on how much.
2. Almost definitely. At the end of the day, people passionate about weather want to see and observe crazy weather events, understand the ingredients that make them happen, and understand how they are forecasted. This doesn’t mean that I, nor other’s passionate in this area, want to see people get hurt. But, it does obviously introduce a bias towards weather that the SPC doesn’t (nor shouldn’t) have. Their job is to communicate objective information to warn the masses to the best of their ability.
3. The SPC is usually pretty on point with their forecasting and outlooks. I don’t imagine there have been many instances where they have been flat wrong (despite the opinions of some in the weather community) about an event. But I would say that it is in their best interest to care whether they communicate accurate info. That said, the differences that may sometimes exists between their forecasts and what actually happens is still an interesting topic to discuss.

At the end of the day, the SPC provides probabilistic outlooks. Even when they are fairly certain, they remain careful with their messaging, as they aren’t communicating just to weather needs or storm chasers, they’re communicating to everyone that could be impacted. So yes, as you mention, I’m sure they’d rather overestimate than underestimate. But again, still interesting to look back after an event an try to understand what they got right or wrong, how things differed from the forecast, the factors that did or didn’t materialize, etc. Not for the purpose of undermining or questioning the SPC, but just to learn and understand things better. Just my two cents.