I think you might want to consider whether 7 is too many, especially online. I'd run different groups of 3 or 4 for a while around Phandalin (you've got LMOP and Icespire Peak for subquests) and then occasionally have an event game (fighting a dragon!) where everyone is on at once. If you're light enough on your feet with regard to plot, you might be able to make each episode enough of a standalone so it won't matter too much if different people phase in or out on any particular night. I've found people don't need strict continuity to cover for absences, everyone understands the practicalities. Phandalin is a good homebase where everyone can live, then set out on an adventure for the night.
Keeping combat moving and fast enough is really hard with new players (and for experienced players) and by the time it's your turn again after 6 other people have slowly decided what to do and asked about the rules again is a real drag.
I'd fight the urge to direct them too much. I know it's very tempting, especially if you have good ideas if they take a particular path and nothing in mind if they do something different. So much of the magic comes from them feeling like they're choosing freely (even if you've subtly loaded the dice: they're often quite predictable given the right incentives). You've got enough to do without being their team leader too. There's a tension here, in that it's important you keep things moving, but I'd do that with your own pawns (goblins attack!) and not by making them do stuff. I do sometimes remind players of cool things their character can do, but they often look disappointed if they feel I'm hinting too heavily. If you feel a player is metagaming because he's heard TAZ, don't stop him from doing something, change things up so he's surprised.
Going from memory / abandoning the source text is totally fine. It's your own unique adventure now. Canon is what you invent in the moment. No one knows the "right" story, and being alive to stuff your players come up with or you think up is usually what people remember with fondness.
1. Let them leave the cave if they want. If you want them to meet Klarg, think of tempting enough bait so you can hook them in. If they do want to rest in the cave, let them, get them to describe setting up camp and then hit them with some wandering goblins, make the place feel alive, new players might find this surprising.
2. I found that combat was the main thing to try to speed up. D&D is very rules heavy in this area. There are a few cheat sheets online to prompt them with their usual actions. My methods for doing it around the table apply less online (basically I want people to know when their turn is coming up, and have their main options on their character sheets). To a certain extent I'd let them be headless chickens. They're low level characters, perhaps they don't know how to efficiently clear a goblin cave, so you can lean into that. You can also ask them in general terms what their character would want to do, and then try and interpret that into actions for them, as a way of teaching them the rules.
3. Talking to players in character yourself is what I try to do here. And if they talk out of character to each other I let the NPCs hear them and react to what they say. It's a bit stressful for new players who don't want to look silly or do it wrong.
5. He was fine for me. Big bads often go down fast anyway (I had a cool blazing rooftop fight and a speech planned for Glasstaff and he got hit from behind with a fireball scroll and never got to speak). BS had some cool abilities (darkness, spider climb) and I had a doppelgänger impersonating a player so it was quite a good fight in the end. I do see people online beef him up in terms of lore (some made him an actual spider / drider), but I didn't think that would mean anything to my players. My players take more pleasure in taking these guys out than they do listening to me monologuing. Definitely cool laying down seeds of potential plot and see which ones spark their imagination, and then take those things and elaborate on them.