My fiance was recently denied a visa with a health/medical issue. he was told that he can reapply in 6 months.

I suspect they were correct when they advised you to really in six months. It probably had to do with the temporary inadmissibility. I am guessing...

If you are a US citizen, you are posting to immigration lawyers who are not in the position to answer this question.

What difficulties will we face

I suggest you get married and apply for I-130. You can do that at the consulate. You will have to convince then you are about to relocate to the USA.

Green card for unmarried children over 21

You must have married the biological father before the child turned 18 for her to be able to do that.

Approval notice i130

You need to schedule an infopass and figure that out. USCIS is clearly confused with something.

I 864 must be sent original or photocopy to nvc???? if i send original, at time or interview consulates will asks again?

Yes you need to send an original and the consulate will get it form the NVC as part of the record.

Family based immigration - preference 2B vs. 4

F2B will always go faster than F4 category. As to the parents, they are immediate relatives so they will be able to come here soon and petition.

More than one I-130 filed for the same person?

It should be no problem. You can have multiple people petition for you.

B-2 visitor visa for unemployed applicant

There is a nonimmigrant intent requirement that attaches to B2 visa application. If he can convince the consulate it would be indeed just a...

(a)(5) is your category. this is the first time I hear that the consulate requires I765. Probably so you would get the card as you arrive to the...