Hello linguaphiles!

I'm back with a few more linguistic dilemmas. I hope
you will be able to help. I'm writing an academic
paper and have just received some feedback on my
current draft. There are a few things I haven't been
able to solve or figure out on my own. Here are the
problems I need your help with:

1.) Consider the following sentence: "The
internationalization theories focusing on
multinational companies are often called the economic approach." My professor has marked this sentence and commented that the English here is wrong. For the life of me I can't figure out what is wrong and why? Can you?

2.) Consider the following: "Europe is often
overlooked when going international. While being
closest and therefore more economically achievable, it
is a mixture of different cultures to master." My
professor has marked the word "achievable" and stated
that this is the wrong word. Why is it wrong to use
achievable in this sentence (I thought it was a good
choice)? Can you suggest better alternatives?

3.) Consider the following sentence: "The purpose of a
case study is not to generalize the results of the
study, but rather to give good theoretical statements
based on the study results." My professor has marked
"good theoretical statements based on the study
results" and commented that it doesn't good. Do you
agree? And if so, how would you formulate this
sentence?

4.) Consider the following sentence: "Besides, every
stage of this process involves a good knowledge of international business environments, such as political, economic, legal and cultural issues." My professor has crossed out "a" before "good knowledge". Is it really wrong to use "a" here? If yes, why?

I really hope you will be able to help and offer your
opinions. I'm looking forward to hearing from you!

Happy weekend, Heni