They are both fine. Not using commas to isolate a word is a suggestion but not the law. However, I do think that using it after the verb comes off as a bit more emphatic. In spoken word more so than written word. "Your son was caught vandalizing school property. Therefore, I was forced to inform the authorities." "Your son was caught vandalizing school property. I was, therefore, forced to inform the authorities." Read these two sentences out loud and you will see that the first seems a bit flatter. The second forces a pause after "was" which conveys more emotion to the speaker. It really depends on what you want the reader to take away from the phrase.

Therefore has 3 different types of usage. - reasoning, attribution and motive. The word "because" can be used in place of "therefore" in many cases. Because of these different usages there is not really a hard and fast rule as to where you should put the word in the sentence order. In the samples that you quote, I would put "therefore" at the beginning. But I think that I would seldom use "therefore" in any case. I hope that this does not make you more confused about therefore. From the rest of your question, you obviously have a good command of written English. However, you can just use "because " instead of "because that"

Avoid using commas to set off a single word in the middle of sentence. Therefore, this is your answer.