1. てくる or てきて is divided to て(and) 来る(come)/来て(come). 
Basically, てくる or てきて means some action and come (to me, here or back here).

So,
> 犬はボールを持ってきてくれました。<br>
> My dog had/got/held a ball and came with it and gave it to me (and I received it).

I think [this link][1] is helpful for てくる/てきて. 

 2. I agree with your friend. It sounds weird.<br>
I think "毎朝、**犬が**ボールを持って**きてくれます**" is the most natural.<br>
First, 毎朝 is every morning, so past tense "もらいました" is weird.<br>
Second, きてくれる basically implies "I receive it", so you don't have to say "私に".<br>
Third, the dog must come to you before he/she gives it to you. So, it is better to say "きてくれる".


 3. 手伝う is not used with "に". It means "help something" though help is used for "someone" in English. It is with just **を** or **も**. like "彼の仕事を手伝う" or "家事も手伝う".<br>
It doesn't have two objects at once.<br>
Then, you need to say "掃除を手伝う" and this cleaning is the friend's task. so you should say "友達の掃除を手伝う".<br>
If you say "友達に掃除を手伝ってあげると言った。" = "I said that I helped him/her clean.", this sounds natural.<br>
 <br>
By the way, you might hear a word like "彼を手伝う". 彼 is not something, but 彼, in this case, means "彼の何か"( his task). Something he has is omitted.<br>
  <br>


 4. くれる(give) basically implies "I received it" who says the sentence. except for question sentence. <br>　
In this sentence, 兄は弟にお金を貸してくれました, who said this is someone but the brothers. Besides, this sentence is not question sentence. So it sounds weird.<br>

For example,
> 兄はお金を貸してくれました<br>
In this case, who gets money is "I" who speaks this sentence.

> 兄はあなたにお金を貸してくれましたか？<br>
In this case, who gets money is "you", not who asks the question.



  [1]: https://japanese.stackexchange.com/questions/83118/usage-of-%E3%81%A6%E3%81%8F%E3%82%8B-in-motion-of-object/83143#83143