He's still predominantly on the right side whether that's out of need or not and it's been that way for a while. The point is that no matter how you slice it, Rielly can and likely will go on that side. If the Leafs end up going with what they have, Rielly still has to go on the right side because they have no options at the top of the blue line to fill that spot.
One, Fowler is not my guy. I hope he is dealt from Anaheim because I know he's a good player and it would make it easier for my team against the Ducks.
Two, Polak doesn't fill a need...at all. Physicality is a luxury in the NHL. Puck-movement from the blue line is critical for success and teams with depth of defensemen that can move the puck tend to perform the best. If Polak is in your top four, which at this point he likely is, your team is in trouble.
If both Zaitsev and Carrick are RHD's, it really has no impact on the viability of trading for Fowler who plays the left side. Now setting aside that that argument isn't much to begin with, you can't bank on either of those two making it and performing where they need to for the team to succeed. The biggest reason why you would take Fowler over either of those two is that Fowler is proven and the others are not and considering Fowler is in the same age range, that's a pretty big deal for a team lacking quality blue liners.
Marincin is a decent option on the 3rd pairing if his puck-moving abilities improve but beyond that, he's a dime a dozen player. There really isn't anything special about him but he's useful in a depth position as long as the puck skills get better.
As for not needing Gardiner and Fowler, I would say that's incorrect. There's nothing about either of the two that makes the other redundant. Puck-movement capability is critical from the blue line and the Leafs have nobody proven beyond Rielly and Gardiner and since Rielly can effectively play the right side and likely will continue to play the right side, that opens up their options to grab a d-man on either side to fill out their lineup.
Lastly, it's funny that you want to marginalize a player and a discussion simply because of a difference in opinion and that means you're marginalizing a player that played the most minutes on the team that won the Pacific. It's really pathetic to go that route.
I would...easily. The impact left wingers have on winning isn't that great unless you're a legitimately elite player which JVR is not. The impact that blue liners have is certainly larger and in Fowler's case, he would at worst be 2nd pairing in Toronto and considering what's there for them at this point, it would be a significant swap for them. Fowler would help the Leafs win more than JVR would. That's the bottom line. You need a solid blue line a lot more than you need a winger like JVR who is a solid producer but not much beyond that.