I have heard that DMs do not want people to use the staircase to escape PvP. Yet, it seems to occur often, so I am unsure if that is the official policy.

1) Are you allowed to use the staircase during PvP to escape PvP?

2) If not, is this because there is no way to regulate it?

It seems like an easy way to regulate it would be to make both sides a placable which can be locked by a DM during PvP!

You can use it to escape, but not to pop in/out of for like, healing/buffing etc.

If you use it to escape, you're escaping and not rejoining.