VPNs are usually public and accessible by a lot of people which are doing some actions (good or bad) which triggers some of the IPs to flagged in public spam checking databases.
And on the other hand, Instagram has a very well implemented spam detection algorithm. If you can register with a VPN it doesn't mean that you're not flagged.
If you have access to dynamic IPs you can create with them, otherwise try your mobile data and everytime you creating an account do some actions manually, sign out, clear cookies and if you can change your user-agent. Finally, activate airplane mode for about ten seconds and do those another time.
Usually, yes, but It's not a must. It's only because that some of the users do have access to dynamic IPs or using their Instagram account from a public wifi.
But you have to care a lot about spam score of the proxy that you're going to use.