Need VBA script to auto-insert value upon row insert

I have a spreadsheet that is tracking ownerships of land parcels for importing into a database.

Currently it has about 300+ records (rows).  In column A, there is a field named Sort_Key.  They are only whole numbers.  The column is formatted as GENERAL, which when using a whole number, would connote a INTEGER format, right?

Anyway, the whole idea behind the sort key concept is to allow me a way to revert back to the sequence that the data was originally entered, in case I changed the way the spreadsheet was sorted.

I have been told that I will no longer be doing the data input on this spreadsheet, and because of that, will be forwarding the task of data input to a much a very inexperienced Excel user. This person will undoubtedly forget to add the sort key value whenever he adds/inserts a new row, I fear.

To avert this potential catastrophe,  the only thing that I can think of is a VBA script that will automatically insert a new Sort_Key value that is +1 (incremented) from the highest previous value.

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The work flow for understanding the construction of the VBA script is as follows:

1 - User initializes INSERT ROW command.

2a - With Excel listening in the background for any INSERT ROW command, the VBA script initializes.

2b - A dialog box will pop up informing the user that upon completion of the VBA script, Excel is going to place a NEW, incremented value in the cell (in Col A) on the new row, for the new Sort Key.

2c - The user will would be prompted to select EITHER: "YES - proceed with new Sort Key Value" or "NO - I don't want to generate a new Sort Key at this time".  If the user selects "No ...", then it will return to the cursor position, and the VBA script will abort.

3a - If the user selects "Yes..", then the VBA script will continue.

3b - The script will read in Col A the MAX value, insert it into the variable to be incremented, then increment it by 1.  The output will be inserted in the new empty cell in Col A.

4 - Once this is done, the script will place the cursor in the next column (which is named: Tract No), and the routine will end.

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Some other questions/comments:

A) What if the user changes his mind and decides NOT to insert a row, and wishes to UNDO the new Sort Key?  What are the ramifications here, if any?

B) I was also thinking of another way to do this would be to have a custom button that is assigned to the VBA script.  Whenever the user wanted to insert a new row, and then add a new Sort Key value.  Since this could be construed as a "deliberate" action, it would do away steps 1, 2, and 3a.

C) Is there a hidden value that Excel keeps that could be utilized (in some way) to satisfy this application?

I hope that I've explained this sufficiently, but if not, please place your questions and I will respond to them promptly.

Thanks in advance.

Phil