I'm gonna try to answer some of you questions as best I can!:)
1. If it's the gunholster you are talkin about, (English is not my native tongue) it goes on your vest slot I believe.
2. This sound alot like the angel arms glitch (also called spaghetti hands) this can happen from time to time, and the solution is closing the game, closing the dzsa launcher. Reopen the launcher, and press "load" instead of "play" on the server you want to join. When the game launches to loading screen, simply press play and that should fix it.
The greyout of the scopes etc can also happen when you accidentally attach more than one scope to the weapon, different scopes use different slots on the modded guns, and does not block other slots beeing used at times.
3. This is sort of a question of personal opinions, there are several big backpacks etc. When I gear up for raids etc and focus on storage I use:
Large tactical belt 70
Mvs rucksack 180 slots
Heavy mvs rig with all attachments ~60 slots
Field jacket and pants ~60 slots
Many of these things can only be bought at trader, or taken from the victims of your brain of terror.
Experiment, find out what you like. It's not always about space. More gear means a bigger silhouette, and beeing spotted more easily.
4. Yes, ammo needs to be stacked manually.
5. Yes, the guns normally dont come with everything fitted, and you will need to search for that in addition to the gun, or buy at the trader.
6. Batteries usually spawns in industrial areas, and on zombies. You can also find them in toolsheds and houses.
7. Might be bugged, did you hold "r" to reload?
Hope this helps a bit. Hit me up on discord or continue this thread if you need anything else.