Are corporate employees not allowed to use personal subscriptions?

Sounds like a bad idea in general. Any data use agreements get lost, shadow-IT brews and nobody knows what tools to use, oh and it's against the service terms.

Unless you get written permission you can be sued for publishing their trade secrets. This can end in jail time if your employer is particularly uncaring.

Ymmv, do whatever you want . It's your life.

Generally not, since the corporate account has all the privacy knobs turned up. I use my personal account on my open source projects, where code leaks aren’t exactly an issue.

If you want your company's code to be used in training, then yes.