SharePoint Sharing Scenarios and how to deal with them

We're trying to get our head around the many ways you can possibly share on SharePoint and trying to strike a balance between conveniency, ease and security - ensuring we're not being too restrictive.

The two main scenarios we're trying to accomplish are:

1) A user shares a file with another user for viewing (read-only)

In this scenario the user sharing the file is essentially "pointing" the other person to the file, and that other person would access it using their existing permission (by default we generally allow "everyone except external" visitor/read access to everything and lock/break inheritance by exception where necessary).

We can easily achieve this through disabling the Access Request Settings in Site Settings > Site Permissions :

However, this unfortunately prevents the next scenario from working:

2) A user shares a file with another user to edit/contribute

In this scenario a user shares a file with another user expecting them to review/edit/contribute/collaborate on that particular file. This seems to be a common requirement and the fact that a Share option is splashed all over the place suggests Microsoft are OK with this method and approach. 

The problem is that many articles & forums suggest this is bad and breaks inheritance etc. etc.

So how best to deal with this situation?

The other problem I have experienced is that I enabled the access request setting to allow for this scenario and it allows me to invite and share a file with someone, but unfortunately it's a bit tricky in that you end up, as a member (edit/contribute) of the site unable to revoke that permission.

Also there appears to be a few gotchas when you hit the Share button where there is a tick box for "Allow Editing" which is enabled by default so it would seem the user needs to manually change the tick box etc. leading to potential sharing mishaps.

Thoughts/comments/ideas on how you've dealt with this in your business would be great!

cheers

Baronne