Hi and welcome to iCraft.
1. I, personally, don't enter a description in the caption box for each individual photo. And I haven't seen too many vendors that do. My suggestion would be to only enter a caption to the photos if you think it will add value to the photo and add value to your entire posting. But you’re right, your description is in the main box and most of the time that's sufficient.
2. In terms of shipping, I know your pain. As far as I understand you have to enter the 3 options again in each individual posting just in case there's an item you don't want to send to a specific location. International shipping can get pretty expensive, especially if you're shipping a heavy item. If your husband makes, let's say a wooden night-stand or head board, the cost of shipping it internationally may be more than the actual price of the furniture, so you may only choose to send it within Canada or to the USA and not internationally. Whereas something lighter, like your placemats, you can choose all 3 options.
Plus, there may be some items that you can't legally send overseas, or even to the US, because of ingredients, components, whatever. Handmade crafts I don't think are a big issue with Customs but you should check with your local post office about items that should not be shipped or mailed, to be on the safe side.
I’ve noticed that you currently have $0.00 for shipping. If you don't intend to charge for shipping, then you should say FREE SHIPPING !!! Bold it, capitalize it, put stars around it, put it in your Exhibit Announcement on your profile. All customers love to know where and how they can save money.
If you will be charging shipping my suggestion would be to replacement the site for other vendors who are selling the similar items as you to get an idea of what shipping costs are, and you’ll learn as you go about what to charge, if you decide to charge at all. Also, be sure to check with your local post office about weight of items you’re shipping and fees.
3. I can see your dilemma with the categories.
Maybe for these types of items (wooden boxes)
https://icraftgifts.com/handmade/32238/wood-keepsake-box-kokopelli-dancing.php, the appropriate category would be Crafts – Woodworking – Wood box. ?
For the baby quilts – Home Décor – Blankets – Baby Blanket or Baby Throw. (But to be honest, I don’t really know what the difference is between a quilt, a blanket or a throw :/ )
For the mugmats and placemats, I can only suggest Home Décor – Dining/Kitchen – Dining Table Top.
I make jewelry so I have not experienced this issue and I’m not sure what the process is or the turn around time. It’s been my experience that they are pretty quick with responses and results.
Good luck.
Bibi