Apple Business Manager queries

Our company currently manage our Corporate iPhone's via a (non-Apple) MDM solution. We have a business app that has to remain on an older version, so has been deployed via VPP with auto-update turned off.

We have only the one VPP account, registered in the old VPP portal (so using sTokens). We are migrating to a new MDM solution and I want to setup the VPP link in it, without affecting the existing MDM (which will be getting retired in a couple of months).

I understand Apple Business Manager is now the way to handle VPP (or as it is now known, "Apps & Books", which use location tokens instead of sTokens).

I was just going to register a new ABM account and "re-purchase" the previously bought apps (they are all free), but I have a few questions first:

1) Can I register (using a different email address) for the ABM account but use the same DUNS number as is registered on the old VPP portal account?

2) If so, when I log in will I be able to search for and purchase the old version of the business app?

3) I assume that, as this is a completely separate account to the VPP account, any location tokens generated in ABM "Apps & Books" will be completely unrelated to the sToken generated by the old VPP portal (i.e. there's no cross referencing of e.g. DUNS numbers etc. between the two accounts)?