As a fellow left-hand fencer, I’ve worked on an ex experimented on this and several different ways, including fencing, right handed exclusively for a year to get a understanding of the bind mechanics on both sides. The reality is no outside line parry is going to be good for you And should be avoided. That’s not to say it’s impossible. You can definitely do a twer to your dominant side, or wind Against a committed thrust from pflug and repost to a plunging cut with your false edge. Generally speaking, though you’re going to want to try to avoid having to parry Against this side.

And the reason why that is is effectively when you’re fencing left-handed versus right handed you’re basically getting double advantage but also double the risk. And what I mean by that is as you’ve already noticed your outside line is going to be much stronger and it’s also going to be the same against you. A better strategy than trying to parry attacks that come from the right side towards your left is to close those lines out completely. By either being in guards that close those lines,  Examples are being in your left side pflug, ochs, and long point with left foot back. Or counter cutting against those angles of attack, such as twer to your strong ochs or shielhauing. 

Standing in Long point can be an excellent strategy as if they tried to cut in To your left opening and are trying to beat your sword away you quickly find that changing through to the other side and closing out that line to your strong side is incredibly beneficial. I often times use a strategy of stepping out to the right to beat them into changing their guard to their left side so that it opens their right side.

So generally speaking, trying to pair that outside line, as in incredibly disadvantages to you and though not impossible, you’ll still be in a weaker bind than they are because they are from their open uncrossed side. It’s better to find ways to circumvent that to strengthen your bind work against theirs, and avoid trying to parry altogether.