Those are just slight corrections and suggestions:
3) I'm sorry, your arguments are unconvincing / tenuous
7) I'm not sure if "serve on" is the most appropriate term when you want to talk about your position in a company, from what I know, this term is used when speaking about working as part of a body that carries out a specific duty or task (for example an association, a committee, a board, a legion, etc.). If you want to use something else than "worked for" maybe you can simply say: "He has served (not served on) / been part of our company for a couple of years."
8) Be careful with tense here: if you want to talk about an action that started in the past and is continuing in the present, the easiest way to do that is to use the present perfect: "The valuation gap between BP and HX has widened in recent year."
10)The spokesman signaled (spelling, only one "l") that both external and internal candidates are being considered.
The rest is pretty solid otherwise, good job Ann :).