Hi,
Please note that the integration between JEvents and HikaShop has been developed by JEvents themselves. We don't know how it works, what it can do or how it does it. I will be best to contact JEvents support in such case.
1. That's something which has to be supported by the JEvents integration and thus, we cannot help.
2. I suppose that JEvents redirects you to the checkout of HikaShop with the product already added to the cart.
If you don't want to allow the customer to change the quantity of the products on the checkout cart display, you want to edit the "checkout workflow" setting of the HikaShop configuration and change the "cart" view to "cart status" instead.
3. If the integration doesn't support it, I don't see how it could be done.
So for points 1 and 3, I would recommend you to contact the JEvents support.