Yes you can check in now.
If you want to make room requests you'll have to call or fax them in since that feature is no longer functional on online check in.
Your room is generally chosen prior to arrival but if you're not a fan of the location you could always ask when you get there if there happens to be something else available.
Sorry not sure how DVC works. You may want to ask in the DVC section.
I just call the regular Disney World phone number and talk to a CM who looks up my reservation and notes my requests.
Yes, if you’re a DVC member you need to contact MS to put in a room location request. You can call or use the Chat function on the member website.
If you check in online you’re eligible to skip the front desk on arrival by receiving your room assignment via email or text when your room is ready. No guarantee that you’ll be able to skip the front desk but if you don’t check in online then you’ll definitely have to go to the front desk when you arrive. (We always check in online and usually but not always get the text.)