OK first bike ever?? Not criticizing but rather expressing a concern. The Venture is waaay too big for a first bike and heavy too! I just hope you don't hurt yourself big time learning how to ride it!
The headlight can be accessed by taking the scoop off of the area below the light, and the bezel around the light. Reach up there and you should be able to reach it. If not both fairings will have to come off, but then you have an excuse to fix the plastic up...
Bulb is a standard 9004 but many of us put in a Silverstar for brighter light. Also, clean the black rubber bezel and with paint for rubber paint it white. Makes a huge difference in brightness!!!
As far as the indicator goes, either the bulb is bad, or your reserve lighting unit has bit the big one. The RLU is on the front fairing sub frame behind the relays.
You are probably right about the throttle cables. You will also find issues with the carbs and they are tricky to rebuild.
Brakes are weak on the 1st gen MK1 (83 - 85) nd many of us have upgraded to the MK2 brakes. You do know that the rear is linked to the left front? I would flush all of the brake fluid from the system, front, rear and also the clutch.
The battery light is most likely a crudded up probe. The light is strictly for monitoring the electrolyte level (acid) in the battery. Many of us switch to an AGM battery and eliminate the probe with a resistor mod. As you will eventually discover there is an issue with hot starts which are eliminated by an AGM battery, bigger gauge battery cables, and a 4 brush starter motor out of a 2nd gen RSV.