手伝って くれて 、ありがとう。おかげで早く終わったよ。
I understand it roughly:
Thanks for giving me assistance, thanks to you, the task finished faster
But why it is くれて not もらって?
I thought the context was (I am really thankful for receiving assistance from you and thanks to you the task finished faster)
Also, the person who assists seems to be outsider, so therefore shouldn't もらって be used instead?
And why the くれて is in て form instead of くれた?
The difference is the subject of the verb. The person you are talking to is the subject of くれて, where you would be the subject for もらって. Because you are thanking the person for the help that they gave you it makes sense for them to be the subject, and so くれて is used. It's comparable to the difference between "you did something for me" and "I got you to do something."
As for why it's in the te-form not the past tense, the tense is expressed at the end of a sentence (usually). Just like how in a sentence structure "he verb-ed, verb-ed, and verb-ed" the first two would be te-form and only the last would be in the past tense, 手伝って くれて ありがとう（ございました）only puts the last part in the past tense as well. Essentially it's te-form because that is a standard way of linking verbs, past tense is not.
