Eminem

The joint sponsor needs enough to sponsor themselves, their dependents, and you (not your wife, she's your main sponsor). Sounds like they don't have dependents, so it's them plus you, for a total of two. 
 
For them, their assets need to make up 5 times the difference, since you are not their spouse. 

That's a good point, I'm not sure at the moment/off the top of my head but I'll try to do some reading when I get on break to hopefully answer that 

Correct - her friend is your joint sponsor, and her friend's husband is part of her friend's household. Friend fills out I-864, friend's husband fills out I-864a in addendum to friend's I-864.
 
Your wife fills out her own I-864.
 
In total, two I-864 forms, one I-864a. All good. 

Yes. For example, if the joint sponsor is her father, who is not part of her/your household, then he can use his income for a second I-864 (your wife is always your primary sponsor and must fill out an I-864), and her mother/his wife can fill out an I-864a, and so can any of their children who live with them, or any other family member who lives with them. You can only have two I-864s (sponsors), but each I-864 can have as many I-864a forms as required, though don't go overboard. 

90 day rule is definitely not a thing. It is also not a USCIS policy but a DOH policy ( I am at work and cant find the source right now, but someone please correct me if I am wrong). 
 
You were inspected and cleared at entry and that clears you from any "intent" issue. 

I would go ahead and file. You can read through VJ, lots of cases of people filing before 90 days and having no issues. 

The 90 day rule is not set but can be use as a guide. 

Your authorized stay on ESTA expires after 90 days. You’ll be out of status if you haven’t filed adjustment by then. Technically you could be deported (though it’s unlikely to happen).

Assuming you had no intent when you were inspected and allowed entry, as you have previously stated, then you are fine to marry and adjust status in the US. 90 day rule is a Department of State rule, but USCIS officers, in their discretion, may use the rule. Overstay of ESTA is forgiven for spouse of USC.

I have heard that this is possible but it is best to wait some time before you get married. If you were to get married too quickly, when applying for adjustment of status, it might appear that you were fraudulent when you came into the country. Since you did in fact change your mind about potentially staying it wouldn't be fraudulent but I have heard that it is best to wait. You probably should consult an attorney. 
 
I am not sure about the income requirements but I do have a friend that sponsored her future sister in law without any problems.

Perfectly okay to marry and adjust status.  You do not have to "wait some time."  That's a myth.
 
Using her mom's assets would mean her mom would be a Joint Sponsor.  If it's her primary home, then she can not count it for the I-864.
 
Anyone who is a USC or LPR, is over age 18, and living in the US can be the JS.

Not aware of anybody having issues adjusting from a NIV due to this, well for a very long time, things were different 10 plus years ago.