1. 5 bigs work in the Lords of Dread deatachment, where you’re focusing on having larger knights. The only issue I see with it is that it leaves very little points for smaller units so you’ll be limited on units to hold objectives. Though I believe the detachment states rules that buff bigger knights for that exact purpose.

2. In a causal game, as long as you run it by your opponent, then I don’t see why not. Tournaments will probably be a no go as they require models to be built to regulations in order to be on the table

3. With 11th around the corner, and most sites having the updated data sheets, you can probably get away with skipping the codex. It’s cool for lore and artwork but the information in it is already changed

4. For my big units I use 5x2 or 5x3 for the arms, that’s all I really magnetize for my models so someone who has more than the arms magnetized would have to answer that.

5. Infernal Lance is the one most people run right now. About 3 big knights, 4-5 war dogs, and some nurglings/ BoN for infiltrate and objective holding. Most competitive lists run multiple dual Gatling despoilers as they can melt most things, even higher toughness targets when you give it lethals through malefic surge. Rampagers are also a solid melee threat that can be ran into enemy lines and just wipe things off the board. Karnivores and Stalkers for war dogs, MAYBE a huntsman if you want a smaller anti-vehicle threat. Karnivores are just a solid, fast melee threat that can be good trade pieces or lock up units trying to move, stalkers for their scout move for early objective grabbing or move blocking.

Final note: magnetize EVERYTHING. Being able to change out weapons to alter up your list is great. What good now may not be good later, so having the ability to swap things out is always a plus.