Excel 2016 - open all files in ONE instance

Hello,

I have a problem with my Excel 2016 (16.0.10228.20051 32-bit):

I have one Excel instance running. (N.B. - the "Ignore other applications that use Dynamic Data Exchange (DDE)" checkbox in Advanced Options is NOT checked.) What I need is - for all Excel files that I am about to open to open in my first and only Excel instanc e, always, and regardless of:

• how many files I open simultaneously,
• where the files are located (i.e. on local drive or on OneDrive so the system needs to download them before opening)

 and including when opening Excel files from other Office apps, i.e. Outlook attachments.

The behavior that I've been seeing in Excel 2016 is, at best, inconsistent:

1. When opening multiple additional files at once from disk (select multiple files/right-click/Open) - it opens the first n files in the first Excel instance, but then from n+1 file it starts opening them in a new instance.
2. A variation of the above is when in my first and only Excel instance I have a workbook open in XLSTART folder. Then upon opening multiple files at once, when it reaches it's n+1 file and the new instance launches and tries to open the file in XLSTART folder again, but it is already open in the first instance, so the warning message saying "this file is already open" (or something like that) shows up and files n+2, n+3, n+4, etc. till the last one, will all try to open in a separate instance, each triggering the same warning message.

I'd like to state at this point that this is not file specific, the file n+1 is usually a different file every time.

Also, I am aware of the workaround (i.e. dragging files onto open Excel window) and I am using it - the above is to demonstrate the symptoms I've been experiencing.

3. When generating Excel files by another application (call it AppX) - you might say that this is likely the AppX's issue - I have no means to verify this unfortunately, however, the randomness of the new instances launching in this case is similar to all other scenarios described here, so I doubt the cause is my AppX.
4. Finally, when opening Excel files from Outlook email attachments - i.e. double-clicking an Excel attachment in Outlook's reading pane - it may open it in the first Excel instance, but may also decide to open it in a new one. From my testing, if the Excel's file previewer is disabled at that point (i.e. listed under Options/Add-Ins/Manage - Disabled Items) it will most likely open the attachment in a new instance.

So that's my observations, hope it helps to shed some light on the issue. I know how to open files in a new instance if I want it. The problem is, it happens when I don't want it and in some cases, I don't seem to have any control over this.

Thanks,

J.