Yes, they could try and get some other random tactcian/diplomat/scout/whatever (damn looking from this angle Rowan is a freaking swiss knife huh), but as you said, it would take a lot of kidnapping and village burning for a lesser result, consuming more time and probably attracting more attention than what they could handle at that point. My bet would be on Andras getting impatient though hahaha.
Kinda unrelated, but I actually count the endings where you fail a deadline from the twins as a "normal end", where Rowan and Alexia die but the twins and their subjects soon follow after their plans are foiled by the kingdom army and possibly some of the other remaining heroes.
Well, let's also remember that Rowan at the start is a squeaky-clean exceedingly pure Hero, with no real ambition or inclination towards personal gain. He didn't have much for Jezera to work with
but kidnapping his wife.
Other tacticians/diplomats/scouts/darkelfbribes? Are a lot more readily corruptable. She could bribe them, she could seduce them, or, yes, just snatch-and-bag them. It wouldn't need to be nearly as much of a spectacle as the had with Rowan; among other things, they wouldn't need to wait months for Rowan to show up when they could take their target immediately.
And while any given person might not be as competent as Rowan necessarily... There'd probably be a lot of gain from not multitasking all the time. How many headaches would be solved by their Seneschal actually being in the castle all day instead of just weekends? A full-time diplomat handling the Orciad situation? Their tactician working more closely with the troops instead of traipsing about the countryside all day? I don't think the difference in performance would necessarily be inferior - just more of a pain to find the right people. But one thing Jezera is certainly good at is finding people.
But considering how much of an effort Andras made to kill Rowan rather than admit that they could
maybe benefit from outside help... Yeah, he probably wouldn't agree to try this with anyone else.
Good question, as I haven't played the infiltration route yet because I had other priorities while researching (Cubi and map to get a better army to clear as many capture events as soon as possible) so I couldn't compare. Has been a long while since I last played but IIRC he takes an arrow to the kn chest from Helayna and has to leave to recover after the battle while Jezera takes over the fortress in the direct assault.
That sounds pretty similar, and glancing at the event text, I don't really notice any significant difference.
I'm not certain whether to crack a joke about Andras here, or a joke about how incompetent the army at Rastadel must have been that they couldn't match the results of a tiny little garrison betrayed by half their members, but... Hey, I get it, bad luck happens. Anyone can catch an unlucky arrow, it's not as though you can see everywhere.
(But avoiding arrows is probably easier if you wear some actual armor. And if you're in an opposing army, you should probably aim for the big red shirtless guy who's killing everyone; you won't
all miss.)