Internally, the database path is stored in one place, and options in another. As many operations from the WebUI needs access to the database, it made sense to just hide the database things from the users.
I have been planning to fix this a bit by adding some logic that checks if the user has changed the --dbpath option and ask them how to handle it, but for now it is a bit obscure.
That really depends on how much data you put in there. It should not grow larger than the size of the database itself (well, perhaps marginally if new data is added).
Duplicati compares: metadata, filesize, timestamp. If either of these values have changed, the file blocks will be scanned. The rationale is that hashing is an expensive operation, and can be avoided.
To get the previous values for this, it looks in the database and finds the previous values for the current file. Some users report that this lookup is slow. The --check-filetime-only makes a faster lookup, by only loading the previous timestamp, and ignoring the other values.
Some people are using programs that fiddle with the timestamps (i.e. TrueCrypt) and these files stay the same size and the same timestamp. You can use --disable-filetime-check to force scanning all the files regardless of their attributes.
That is block lookups, it checks if it already knows the data that it found.