1) Yes, you can build locally using the PhoneGap CLI. This is not officially supported yet, but I think we will add support at some point, since it allows for any size build and also circumvents the subscription for PhoneGap Build, but it's a lot more complicated. You should be able to choose a normal PhoneGap export from C2 and then follow existing PhoneGap documentation on how to build it offline.
2) Looking at your link, the issue is already marked as 'fixed'. Hopefully it's only a matter of time for this to come through to the stable build, but you could possibly get the fix earlier by choosing the beta version of Crosswalk. (You'd need to check exactly which versions the XDK is using.)
3) All it has to do to show the C2 loading screen is load the html and parse c2runtime.js, so that should be no more than a second or two's work, so a 30 second wait is definitely suspicious. If you've got Chrome's inspect tab up then maybe that time is spent trying to establish a connection? I don't know what it could be doing for that time, but I haven't heard of such a long delay before even the C2 loading bar appears before. Note currently on Android Chrome (and Crosswalk, consider them the same thing) is slow at decoding audio which can add to the loading time if 'Preload sounds' is on, so it may be worth turning it off and seeing if that helps. The linked bug report shows that Google are aware of that particular issue though so it may be fixed in future as well.