A few points, which will hopefully clear things up:
First of all, a cellView can only be opened once in memory at a time - there's only ever a single cellView ID. So that means it's always in a single mode - if you open a view first in "a", then separately in "r", it will still actually be in "a" mode (for example).
Scratch mode was not initially supported in OpenAccess versions, but this was added to enable compatibility with previous code (it was added in an IC612 Hotfix, so it's been there in all IC613 and IC614 versions). It was also there in CDB.
There are some differences in the way things worth between CDB and OA. In CDB you could not modify the data in a cellView opened in "r" mode; if you wanted to make changes, you needed to open it in "w" (overwrite), "a" (append/edit) or "s" (scratch) mode. Scratch mode means that you cannot save the results, however. In OA, you can actually modify readonly cellViews, but again, you cannot save the results. So strictly (I believe), "s" mode isn't really any different from "r" mode (I've not really checked in OA-enabled versions).
Note that cellViews opened in scratch mode must exist on disk - although they might be empty. Currently there's no way to open a scratch cellView in memory only (although the edits themselves are only in memry).
Now about "context" cellViews (referred to in the "sd" and "wd" and "ad" open modes. These are not really used any more - they were in older software versions. So I wouldn't worry about those modes - I've never had to use them.
The "wc" and "ac" modes are to do with creating database representations alongside (say) textual views. So for example, if you have a Verilog textual view (e.g. a "functional" view, you'll see there's an "OA" representation alongside it). This is a non-master database - and needs to be opened in this "c" mode.
For scratch cellViews, it's almost certainly not important to know about the "sc" mode.
So the best way is if you want to go into "s" mode, you can do so, and then use dbRefreshCellView() and dbReopen(cv "r") once done. Or dbPurge() to purge from memory.