Hi neptune,
1. We can’t tell you that, it depends on a huge number of factors. The amount of server resources that are available (are they fixed? Burstable?), the number of peak vistors/sec, the PHP runtime version, whether caching is in use (just a caching plugin, query caching too? etc) all impact here.
2. I would not expect that having a large number of events would impact on upgrading, though if there was a major schema change possibly it would introduce some challenges.
3. No word yet, but stay tuned!