Hi Katie. You can add/remove and edit your store categories starting in your seller hub. Click on your store icon and you will see the first shot. I've done three screen shots for you. Hopefully I upload them in the proper order. I'll see what I can do to help you with your second question in another reply.
Click on Manage my store.
Then Store categories to the left.
Then add/remove categories.
With your other issue go the Help & Contact at the top of your page and Type in Management.
Select Setting Buyer Requirements
Select Buyer Management
Change your Block Settings to what you want.
It's possible that your buyers are getting that message from you because you have blocked them.
Coupons will also limit sales to one item per customer. You may want to check in your coupon section of your store to see if you have anything active.
If your listings are not accepting multiple quantities of an item being available, that sounds like a glitch with ebay. Other than coupons, I don't believe there is any way to limit items to one per customer, and I just called ebay to confirm that (but they are sometimes wrong). Hopefully someone else has an answer for you on that, but to me it sounds like a glitch.
That is not at all what I see on my store. I cannot even follow your screenshots after the first one. It looks nothing like that at all. I have attached a screenshot of mine as well. I do not have a place that gives me an option to manage my store. I did get into my categories list, but cannot arrange anything. It is all arranged by the auto options and is what I want to do is move specific faith based items into their own category.
OK, great. It sounds like selling limits are not getting in your way.
I am not sure why you cannot change the quantity. Here is another way to update the quantities for a variation listing that you can try (if you have not already).
From your Seller Hub Manage Active Listing page find a listing that has the wrong quantity. Look below the title for something that looks like this [XX Variations] indicating how many variations you currently have listed. Click on [XX Variations].
The next page that opens will show you all the options for the listing, along with the quantity of each. Hover over the quantity for one of the options and you will see a little pencil appear. Click on the pencil and a box will open where you can edit the quantity for that option. Click submit after you enter the correct number. You can repeat the process for each option that needs the quantity updated.
Here is a couple of screenshots that I hope helps
