Why did Faralyn need the death of a planeswalker in order to get to Shandalar? If Shandalar was a rogue plane weaving in and out of the Shard, why couldn't they just planeswalk to it while it was in the Shard, and then wait for it to exit?

Did Tevish Szat and Leshrac know Faralyn's plan beforehand? Some of Szat's dialogue seems to imply that he knew (or had guessed) what Faralyn was doing, and thought Leshrac knew too. But Lesrac seems incredibly desperate for Faralyn to tell him how to escape the shard, and is as much in the dark as Taysir, Kristina and Freyalise. He could be acting, but, it's rather odd. And anyway, why would Faralyn tell Szat and Leshrac his plan?

The MTGS wiki seems to think that the entire point of Leshrac aiding Lim'Dul was to aquire an army to invade Shandlar with. But this would take a significant amount of long-term planning, in which Leshrac knows that Shandlar is coming long before the Summit, which...doesn't really seem feasible to me. Is it possible that Leshrac has different plans for the army, and then changes them once he has access to Shandalar?

It's quite possible they both know that Faralyn has a plan to get them out and conquering Shandalar, but not strictly what the plan is.

I really need to get a copy of The Eternal Ice. According to the synopsis of it, Lim-Dûl had Jodah researching Shandalar's approach, and then in the comic, Szat quietly asks Leshrac when Shandar will be arriving. So it seems they had a fair idea of what was going on with regards to Shandalar, they just weren't sure how to get onto the plane.

I wonder if Shandalar didn't actually enter the Shard, but kind of, bounced off of it (the referred to 'juncture'), and Faralyn was able to use Chromium Rhuell's life force to boost his walking and punch through the Shard as the juncture happened.

I'm pretty sure The Eternal Ice implies that Shandalar actually entered the Shard and no additional hole-punching was necessary. When Jodah tells Freyalise of Leshrac's escape plan, she says something like "He wants to use Shandalar as a boat then. But he's a fool. He should be using it as a hammer." (not an actual quote, just from memory), because she realises she can use the impact of Shandalar passing through to cast the Worldspell and break the Shard. Leshrac just comes to Dominaria to pick up Lim-Dûl when Shandalar is close enough. I suppose he was going to take Lim-Dûl's army of zombies with them (no idea how he would have done that, though), but as that was already destroyed at that point, Leshrac was pissed and told Dûl he'd have to build him a new army on Shandalar. So I guess the part where Faralyn needs the death of a planeswalker doesn't really fit in there anymore, unless they were going to use it to move Lim-Dûl's entire army between planes. I think that might be the most reasonable solution as it explains what use the zombie army on Dominaria would have been in conquering Shandalar and would allow you to still use the 'death of a planeswalker needed to do XY' bit.

I'm actually not even sure if the timeframe of the novel would allow the Summit to take place at all, though I can try and research that when I'm done with more pressing meatspace stuff in two days or so...

Yeah, the summit kind of seems...unnecessary. Unless Faralyn was trying to get to Shandalar before it entered the shard. If it was close, but hadn't entered yet, and he wasn't sure if it would survive hitting the Shard barrier, or if it would bounce off and go away... Hmm.