He's more likely to screw the Dems than the GOP if Gary Johnson 2016 is any guide. An anti-establishment Indie trying to out populist Trump would hurt him the most.
Gary Johnson took more GOP voters than Democrat. Gary Johnson voters voted downballot Republican by a margin of 20% more than they did Democrat.
Jill Stein, however, probably took more Democrat voters than GOP.
Yes, but they were the quintessential Clinton-Republican demographic. Do you really think Johnson voters in PA-06, NC-13, AZ-09, etc. would have broken for Trump? Johnson gave disgusted Romney voters an out and prevented Clinton from swinging enough of them in enough key places. He's probably the single greatest reason Trump won.
Perhaps, but all of this is just conjecture. We have literally no way of knowing what these voters would have voted (or if they would have even voted at all) if Gary Johnson was not on the ballot. Hillary was pretty toxic for a lot of republican voters. Every person that loses in a fairly close race with a third party loves to blame the third party for why they lost lol.
The only thing you could I guess say for sure is Gary Johnson's presence may have slightly improved the GOP in house/senate races, as maybe some of the Gary Johnson republican-leaning voters would not have voted had he not been on the ballot.