What would this be called? Would it be called a pick 8 or still just a pick 6? ?

It is still called a pick 6.

pick 6 and 2 point conversion

No, an INT returned for a TD is a pick 6 because THAT play scores 6 points.

Then, by rule, the team that scores the TD gets to run a PAT and can either kick it for 1 point or attempt to get the ball in the end zone again for 2 points.

So, regardless of what happens on the PAT and wither the end result is 6, 7 or 8 points for your team the actual interception and return scores 6 points.

After all, you don't call it a "pick 7" when your team kicks the PAT for 7 total points.

Any interception returned for touchdown is still a pick 6.  PATs and 2 point conversions don't factor in.

Is it a pick 7? No? Than it wouldn't be a pick 8, would it?

The TD is still only 6 so in my opinionit is a pick 6.

A pick 6 is a type of wager offered by horse racing tracks. It requires bettors to select the ... The pick 6 has its roots in the daily double, the first so-called "exotic" wager ... as the exotic pick 6 pool sheds tickets, they have several tickets that are still ... pick the other four winners will win or share in that day's six-winner pool, but ...

pick six. Regardless of extra point attempts, the Touchdown is still worth six.

They call it a pick six because a touchdown is worth 6 points.  The extra point or 2 point conversion are separate plays, and the person returning the interception may not even be on the field for those plays.  

I guess pick 8, since pick 6 is only used when the team misses the extra point.  Most of the time it's called a pick 7.