No not all ports, just power and and auxilary, but I honestly can't recall a single time that I've lost a usb port.
Here's the (excellent) repair docs if you're interested: https://dl.dell.com/topicspdf/xps-15-9560-laptop_Setup-Guide_en-us.pdf
My old latitude from 2011 is similarly easy to repair, it's running a jellyfin server rn.
I haven't done any real tests so I guess I'll defer to you on that. It's been more than three years of daily use since installing the laptop battery though, so I'd say the fact that I haven't noticed a significant drop in battery life means that $50 is no less then a fair price.
Ok I just looked up oem, damn I'm cheap lol: https://www.dell.com/en-us/shop/dell-6-cell-97-wh-lithium-ion-replacement-battery-for-select-laptops/apd/gpm03/pc-accessories