(All of those are really reasonable things to flag and not unproductive worrying at all! Maybe Blai only does that when it's not a terrifying emergency and he's run out of emergency-level problems to try to anticipate and needs something for that mental muscle to do instead. Anyway, Seldan is fortunately very good at quickly cataloguing half a dozen different lines of thought and adding them to a mental notes-slate.)
Seldan has never personally tried to question someone under a maximally restrictive set-command because, well, in his day there were two Mindhealers in the entire Kingdom of Valdemar and their job was, like, seeing patients with mental problems and not using Mindhealing offensively in combat. But he does seem to have retained some context on what set-commands do, for some reason, including the fact that depending on the person they are varying levels of impairing for having thoughts. Maybe Nayoki has relevant experience interrogating people who are impaired at having thoughts but he certainly doesn't, and it might cancel out the effects of a coercive Truth Spell if "answering questions" counts more as an action than just having thoughts.
Probably everyone who comes with them should just be a Mindspeaker, he can't imagine Leareth has a shortage. Seldan has the attentional capacity of a Groveborn and can handle a dozen simultaneous conversations if he needs to, but Mindspeech is just incredibly useful in general. (He wasn't a strong Mindspeaker as a Herald; he's not sure but he thinks he might not have had any strong Gifts at all, not all Heralds do. He is LOVING his new capabilities.) Blai learning a few words of Valdemaran does seem generally useful but probably not a priority in the next candlemark unless he's really good at languages and can pick it up without too much repetition.
Brightstar is with Jisa and should probably not be transported anywhere. Based on Jisa's report, he's probably not going to cause any problems in the short run.
He's never heard of excessive Gating causing a hurricane or tornado; this isn't the season for tornadoes anyway. Admittedly he's never encountered this many Gates being cast, but it might actually be less risky now that it's been several days and any weather systems ready to move in have already done so.
Iftel has a representative in Haven whose staff are handling communications with Iftel proper, he thinks? This only came up in passing as part of the background Companion-gossip and he wasn't yet viewing it as something to actively gather information on, which he's irritated with himself about how. ...He does think the representative might be a high-ranking priest of Vkandis from Iftel's church, which is...possibly not great...
He has absolutely no idea if blocking Gates is possible! It doesn't seem like it should be possible but Leareth would be the one who knows. He'll bounce that question over, in case Leareth is less able to untangle it from all the other thoughts flashing by.