If you stick with it, yes to the Orange Seal.

Big cuts are big cuts, whether you're running tubes or not; in my experience, 1/4" is about where even a good sealant is going to struggle, but those are few and far between for me, though I just had one today. My cut was big enough that the first "bacon strip" wouldn't stay put at pressure. I tried again, this time tying it in a knot before jamming it in to the cut - with the knot inside of the tire, it held pressure and got me home just fine.

No experience with Mavic tires, so I couldn't tell you what to expect with regard to durability, but it's concerning to me that the punctured area got worse. Approximately how big was it in the first place? If there's bulge at the cut/puncture, I'm binning the tire regardless.

As far a your rate of flats... depends. If you're usually riding 1000 miles per year, I'd probably go with tubes and live with the 1 to 2 flats. If you're riding 3000, 4000, 5000+ miles per year and would see as much as 10 flats per year? Tubeless all the way.