Is it safe to park ships in a player owned structure (not owned by me or my corporation) in high sec?
Can they be destroyed or disappear if they don’t pay whatever they need to pay for the station? What happens to my ships if a player owned station disappears?
Reasonable safe, if you accept that in case of destruction or (non-negotiable) loss of access you have to wait a week to get your stuff back via asset safety.
In highsec you can easily keep the stuff you only use occasionally at an NPC station. Keep your day-to-day stuff wherever is most convenient - which may be a player owned structure. Asset safety will protect your stuff but there is a fee to recover it. Up to you to decide if the convenience is worth the risk.