MTO and access to on premises file system

Let me preface this by saying I'm still fairly new to 365 Admin (it's been a steep learning curve) and haven't even got my feet wet with on premises stuff as yet.
Also, I think some of the admin decisions made previously by others may have been based on just repeating what was found to work the first time rather than necessarily a deep understanding of the best solution.

The situation when I arrived on the scene was this (actually it was a bit more complex and messy than this, but this simplified description covers the salient points at this stage)

One tenant, with two domains, call them old-domain and new-domain.
Two types of user, who I will refer to operations and corporate.

An on premises Active Directory system running a file server. Well to be more precise on three premises  with mirroring of data and a DFS, but from the user perspective when you're one of the office locations and connect to the network the same folders are available to you. Everyone was using Azure Joined Company Laptops to do this, so their laptop logins were also their network logins.

Outside of the offices people connected to the DFS using a VPN (with three gateways in different countries).

Operations Users had one account,  @old-domain, this was licensed for 365 and had a mailbox associated with it. It was also synched to their on premises AD account

Corporate Users had two accounts, one @old-domain with no license, synched to an on premises AD account.  The second was new-domain with a 365 license and mailbox.

If you're scratching your head wondering why two accounts rather than assigning the new-domain email address to the same account, I can't give you a definitive answer as I've never been given one, but for whatever reason when new domains were brought into play on corporate name changes the admins gave them new mailboxes rather than simply aliasing email addresses to the same mailbox (some people had three accounts as a result).

What I did note was that when a new Corporate user was added the admins gave them both of the above accounts, I was told that the unlicensed old-domain one was required for the access to the DFS.

Now for reasons not worth getting into here, a decision was made to move the Corporate users to a new tenant, along with new-domain and then to link the two tenants in a multi-tenant organization. It was also decided to leverage BYOD for Corporate users, so their devices will only be Azure registered.

This has been done, there was some pain thanks to the reluctance of Microsoft applications to switch to the new account locations rather than redirecting back to the old tenant, but that's been sorted.

So right now Corporate users still have two accounts, but on two tenants.

On the Old Tenant they have their @old-domain account, no license, no mailbox, synched to the on premises AD (as before)

On the New Tenant they have their new-domain account. This is where they actually do their work, and is the only account anyone should be communicating with internally or externally.

Access to the DFS is being done using the VPN with the on premises credentials associated with the old-domain account.

In terms of functionality, this works perfectly well, people across the two tenants appear in each other's address lists, they can chat and share information etc. Everybody also has access to the folders they should have access to on the DFS. However there are two issues.

The first, and most detrimental in terms of just getting work done is that users in one of the overseas offices have found their access to the DFS has slowed considerably, despite being in physically the same location as the data.

I believe the problem is that although the data is on-premises, the VPN gateway is not, therefore data does a round trip from the server, through that gateway IP address at the ISP and back to the user. Since they are in a remote location with poor internet this slows things considerably.

So the first question is, how do we take that loop out of the equation so that when they are in the office they connect more directly to the servers on site? Ideally without having to revert to needing an Azure AD joined device.

The second issue is that those remaining old-domain accounts (the ones for the Corporate users who are now working on the new tenant) on the old tenant are messy, in two ways
1) From an admin perspective, because every one of those corporate users still has two accounts, their local one that is synched to On Premises AD, and the the external account shared from the new tenant as part of the MTO
2) From a user's perspective. For reasons that I cannot fathom (but this is coming direct from Microsoft after many attempts on my part to find a way) it seems that while you can control which licensed accounts appear on Teams search by controlling whether they are in the GAL and setting the appropriate switch in Teams Admin, all the unlicensed users appear whether you like it or not. The net result is that when someone on the old tenant starts typing in a name of someone in Corporate, they get two suggestions coming up.

So the second question is, are those accounts actually necessary?