If your web-based product is not free open-sourced, yes, you may use them.
Yes, you need to obtain a license for each developer who works with icons. You may purchase mulitiple single user license or a site license. A site license may be used with unlimited developers.
Yes.
Yes. But those modified icons may only be used within your own non open-sourced projects. You may not sell, resell, or publish those modified icons.