1. Look at what is available. Disney can only give you what is available.
2. Sometimes yes, and sometimes no. Seems they are filling up more now that the rates are posted beside regular rooms.
3. yes, you would only pay the rate of the room you want. However, if you have a discount code now (or then) only a certain number of rooms are allocated for each discount, and they are only available certain dates. Also, if you have a package deal (room+plus ticket and maybe +dining plan via WDW), then you have to consider that as well. It is possible to book a room only (buy tickets separately), two room only , or a package/room only combo. (If you currently have a package deal, you'll want to keep the package for the first part, and make the second resort room only). Disney makes it cost prohibitive to book two packages, you're far better off doing the second hotel as room only.
When you book a package, you pay $200 now, with the balance due at 45 days, and penalties for cancelling closer than 45 days. If you book room only, you pay one night now, with the hotel balance due at check-in. You can also cancel up to 5 days before you go with full refund. Often that's the better way to go, IMO.
If you simply modify room only, no changes. If you change your reservation from a package to room only- technically Disney has to refund your deposit- then recharge it, but you probably won't notice, unless your cc is about to close for the month.
There are more detailed explanations available on the parent website of the DIS.
We like split stays. It takes a little creative packing, but with CL, splits = new lounge, new resort, new pool, different park proximity. Disney will also transfer bags for free, you just have to use bell services. (and maybe lose half a day of room access)