Neville Hobson blogged today about writing-style guides. And, for those of us with a passion for the written word, we can't get enough of them.
The average business person, though, probably has a different view. This is the essence of the comment I wrote on Neville's post:
People in business want to communicate effectively. Unless they can gather their wits in the first place, then no amount of style is going to rescue them.
Assuming they have got through the wit-gathering stage successfully, they then don't want the hassle of reading fat style guide books. They'll end up confused and worried. Their focus could easily drift away from what they're trying to write to how their writing will appear to the erudite. What matters is how it appears to their target audience.
You're right to distinguish between informal 'social' writing and 'business' writing (thankfully, you've avoided 'academic' writing) but it's probably better to encourage people to shed their inhibitions, than to create new ones.
The bottom line for your readers is, "Don't be intimidated by grammatical rules and the like" and, "Your value is in knowing your subject matter and how it benefits your audience".
If you can speak clearly, then (IMHO) you're ninety percent of the way to being able to write well.
(I will confess to a small amount of editing as I read through the above. The version on Neville's blog was straight off the cuff.)