I see no reason why the original tyre can’t be adequately repaired. However, if I had an event coming up like the OP, I would be tempted to put on a new tyre (finances permitting) and keep the repaired one as a spare or as a replacement for when the new one wears out. That would be for peace of mind, nothing else.
Sealant by definition is a “stop gap solution” and many people are riding around with holes in their tires they are unaware even exist. Patching a tire is a solution that should easily outlast the the tire itself.
For the gran fondo, put the new tire on. Patch the old tire and use it later.
In the past, before tubeless, I fixed many a small sidewall cut like that with some super glue and a boot glued to the inside. On tubeless I’d use a patch. Super glue though is still a great idea to hold casing together.
One thing I’ve learned with tubeless on my gravel bike is that one should go for a plug sooner rather than later. Don’t let all your air and sealant leak out on to the road and then decide as a last resort to use a plug.
If you have a squirter, pull over, point it toward the road, and then put your finger on it. Let a tiny bit of sealant come out at a time and see if you can get it to coagulate and seal. If it’s just not happening then plug it. Once you have practice with plugs, you can do them in seconds. Get them deep in and twisted a bit and they won’t pop out. You can do this without even losing all your air.
A quick update.
Zipp have transferred the question to the tire team and I’m waiting for an answer. They have confirmed that using a tube is acceptable for emergencies on that tire / rim combinaison. However, they haven’t specified PSI to use. I’ll follow-up.
Regarding the puncture on the sidewall, I was doing bike maintenance in the garage, and all the the sudden the plug of scellant popped-out, only to reseals within seconds. I think this was a sign.
The reason why I was reluctant to purchasing new tire is availability. They are back-ordered and expected in November.
I’ll try to keep this updated so everyone can benefit.