Re: deb-src
To actually access the deb-src line you'll have to use
apt-get source package-name
The
fakeroot apt-get install package-name
will only download and install the binary from the deb line, deb-src isn't used for apt-get install.

So:
apt-get source package-name
cd package-name (well, package-name with some version no etc.)
fakeroot debian/rules binary
(there are other ways to build, this is one of them)
You'll end up with a .deb in the ../ directory
This one can be installed with dpkg -i package-name*.deb either in scratchbox (use fakeroot then) or on the tablet (as root)