a) yes, in the Reload section you can provide User and Password to have the update-process having access to your file.
This way was reported not being reliable, so ...
b) you need to ensure that the account under which your server/ your services is running has access to your file as well, i.e. the account should be listet in your section access list of users/ admins. The account can be identified by various means, the most obvious is the NTNAME.
Have you tried this already?