Good catch on both. For point one, I just removed the nil default. In my refactoring I undid almost all of these setting record fields to nil because its fairly silly to use a nonstandard value for undefined when the standard value is literally, undefined. One place I didn't change this yet was for the qserver variable because of how couch_query_servers:stop_doc_map is written and I was wanting to minimize the scope of these changes.
As to 2, I added the file removal in the cancellation function. One thing this doesn't address is a generalized method for handling the new compaction lifetime model. We'll have to revisit this a bit to try and get a better abstraction for compaction.
The timing tests I was running were against the views in  which you've used before for these types of tests. The test is just a simple "time curl http://127.0.0.1:5984/indexer_test/_design/test/_view/view1?limit=0" or w/e the URL is for that design doc. Fairly consistently the new-views are a minute or two faster than trunk. So, nothing super improved but convincing enough to me that I haven't trashed performance.
Also, I did spend a couple days profiling the view engine and comparing a couple dirty hacks. I'm convinced we can shorten these times even more with some more refactoring and playing with some other optimizations but they were leading me deep into the couch_query_servers and couch_os_process. I plan on backporting some refactoring for these parts next and that should set us up for some of the more advanced changes to the view engine.
I'd agree on turnign couch_index_api into a behavior but I decided to delay that until after we update the various applications and build system so that we have a proper ERL_LIBS directory. For behaviours to work they need to be importable and that's not the easiest given our current structure. Though it'll definitely be something to add in the future.
For supervisors I think that could be a good idea, but I'm not the most familiar with them and their side effects to know if they'd be a good fit here. You mentioned on IRC that you'd take a swing at implementing that and I'd be quite interested to see what you come up witih.