1. Why is there a financial account field in the timesheet form? It is a compulsory field, yet I don't want to use it. Yet when I do use it, it doesn't seem to affect that financial account balance anyway? So what's the point of it?

2. Is it possible to do an invoice based on a timesheet, but then also add on to it? In my industry it is common to have "Write ons" and "write offs" to billable hours if there has been a pre-quoted price.

Create an employee :

Human ressources>Employees>New employee

-use the 'related user' field to link him with your user
-product (timesheet tab) :select a product
-Analytic Journal:(timesheet tab):Timesheet Journal

Create analytic accounts.

Financial management>Config.>Analytic accounting>Analytic accounts

Fill your timesheet

To invoice:
Select the account (Financial management>Charts>Analytic Chart of Accounts) and you will be able to see analytic entries (the lines you enter in timesheets) and create invoices (use the link on the right).

But does anyone know why they have the financial account field, shouldn't there just be an analytic account field?