What happens if Hillary has a stroke before the Election and dies...does Kaine become the Nominee?

That would be the best thing that could happen for this country

No. The party's national committee would choose a new nominee. The could choose Kaine or they could choose somebody else.

Same rule applies to the Republicans.

Technically they could also re-convene the national convention but in reality there wouldn't be time for that, so presumably they'd just have the national committee choose.

But of course it's Trump who is more likely to have a stroke. Hillary is fine. Trump is an old man with funny-colored hair and a hippie "doctor" who spent 5 minutes examining him, and his blood pressure is obviously very high.

No. "the Democratic Party's charter and bylaws state that responsibility for filling that vacancy would fall to the Democratic National Committee, but the rules do not specify how exactly the DNC would go about doing that. (Congress could also pass a special statute and push back Election Day, giving the dead candidate's party time to regroup.)"

I think the DNC would have to hold an emergency session, with all the delegates, and the VP nominee, to hash things out. It's the only logical thing I can think for the Democrats to do. To the best of my knowledge, this has never happened before, to either party.

The DNC handpicks the successor to Hillary if she dies before the general election.

Should any candidate die in the run up to an election, their party nominates a new candidate. That candidate need not be on a current ballot for any office.

Petah gruffin becomes president

Sure.