I don't consider 5 to be large, by any means. I consider 4-5 "normal" for a game. 8 is large. 12 is large. 5? 5 is average, IMO.

As to your specific problems: Mr Not Interested-Can't-Make-It can just see himself out. I'm more than happy to tell players who aren't interested in the "group" portion of the game that they are welcome to leave.

As to scheduling: pick a day and tell everyone to deal with it. They can either make time and show up or they can not. Every single session shouldn't be a cat-herding effort to find a day that works. Get input on the day most people can make it and tell the rest "this is the day". I get it, life happens, but "life" shouldn't be happening so often as to force reschedules or cancelations every week. Spoken as a 33-year-old with a full-time job and other responsibilities.

On spotlight: take turns. So all your characters are charismatic. Present situations that cater to specific elements the other characters don't have. Got a Ninja? Have a league of assassins that will only deal with him. Got a rogue? Have a theives guild that will only deal with him. Got a fighter? Have a trustworty captain of the guard who will only deal with the him. So on and so forth.

Lean on the idea that each character's mental skills, even if all equal, represents different speaking eloquence on different subjects. Lean on their knowledge skills as a reflection of what they can speak well about, and what they can't. If the Fighter didn't put points in Religion, but has a great diplomacy, he lacks the vernacular to speak well on the subject of religion, or to the religious. If the Sorcerer didn't put points in arcana, he may be very personable, but he doesn't understand magic in the way a wizard might, and can't speak about the math and equations and formula to a wizard in a way that the wizard may appreciate.

Also: enforce the other players being quiet while someone talks. If Bobby is talking to the NPC, Jack and Susie need to be patient, wait their turn and if they want to add something to the conversation, do it after Bobby finishes. If they are helping Bobby then it needs to be made clear who's doing the talking, and how they're helping. IME: once you start getting a flow of players taking turns even doing the same thing, they'll start going "Oh I did this last time, you can do it this time."