Lighting wiring questions 101
Forget switch loops, those went out years ago. If your wiring layout would be happier with &quot;cables running off to switches&quot; like a switch loop used to do, pick up some 14/3 cable to run neutral and hot to the switch, and carry switched hot back from the switch. You'll need that for cabling between any three way switches anyway.
1: Code doesn't give a fig if you run a dedicated lighting circuit or not, so long as you meet code minimum provisioning. My opinion having grown up in a house with shared receptacles/lights is that a dedicated lighting circuit is far better, people who have not had that experience often think otherwise. You do need to provide 3 VA per square foot for &quot;general lighting and receptacle loads&quot;, but code does not specify how you split up that 3000 VA (25 amps) for your 1000 square feet. These days you probably want a good deal more for receptacles and don't need a huge amount for lighting unless you abhor efficient lighting.
2: The required 20 amp bathroom circuit cannot* be used for lighting. (*if it serves only one bathroom it can)
3: You don't need a separate circuit for a GFCI. Just put a deadfront (no outlet) GFCI on the bathroom lighting feed. That can be fed from the lighting circuit if you choose to use a lighting circuit.
4A: Code doesn't give a fig how you arrange the wiring, just that it meets code. You can wire it 17 different ways that all meet code. None of them are more right than the others. What makes sense tends to depend a great deal on the actual physical layout of your walls, switch locations, and light locations. Lights need switched hot and neutral. Switches need hot, neutral, and switched hot. How they get there is up to you.
4B: You CANNOT pull from the dedicated countertop circuits NOR from the dedicated laundry circuit. Those circuits (and the bathrom circuit) also do NOT count towards the &quot;general lighting and receptacle loads.&quot;
5: see 4A.