You CAN use a plane to carry passengers for business purposes as a PPL only when the flight is incidental to the business. Taking a coworker to a meeting that you both need to attend in order to see a client with which you have (or are seeking to establish) a business relationship is a valid, legal use of an airplane within the confines of PPL Privs & Lims.
As you describe it (your purpose is strictly to fly the plane) this is a charter, plain and simple.
Now, if you ALSO had to meet this client you would be able to demonstrate a legitimate "common purpose" for the flight and you'd be fine. Use of a plane is no different than giving your buddy a lift in your car, on the handlebars of your bike, or any other mode of transportation PROVIDED your "value-add" is not just as an airplane pilot.
If you have no business other than just acting as the pilot, you are violating the regs. Read the Haberkorn Opinion and go from there.
Question 2: use the IRS guidelines for reimbursement. They're reasonable.
I would not expense any of it through the company unless I talked to a tax lawyer first. I'd just write the portion of hours flown for work off on my personal income tax return (fuel, fees, insurance, hangar, etc)...but that's me. Again, talk to a lawyer.
Question 3: no. The mere fact that you're earning flight time crosses the compsesationcompsensation threshold. The feds will make that violation stick...they've done it before. Earning goodwill from the boss may also be compensation (as mentioned in another answer).
Question 4: that just makes it seem more like a charter. The fewer aspects of the flight that you, as PIC, control the closer you get to charter territory.
Question 5: yes. You can't be fired for refusing to break laws.