it is normal and if guarantor is a good one.

1）我需要给毎个人申请表吗？包括那个实际住在这里的二儿子. yes

2）这是否属于co-sign的情况？Yes

3）有什么样的risk吗？

normal rent risk.

You use the same way screen all tenant and guarantor. If teh guarantor has good credit, income and the best his own house. This is a good deal.

But still screen real tenant to see. credit, criminal history. Eviction record.

The guarantor must sign the guarantor form in front of you with verified ID. The best is to let him sign with an notary.

I have seen people use fake ID or others to sign doc and later you can not force the guarantor,