You can use Finder's open command to open a file in the default application:
tell application "Finder"
open POSIX file "/Library/Desktop Pictures/Abstract.jpg"
end tell
default application of (info for)
gets the default application:
tell application "Finder"
default application of (info for POSIX file "/Library/Desktop Pictures/Abstract.jpg")
end tell
You can use perl to get the default application for a URL scheme:
VERSIONER_PERL_PREFER_32_BIT=1 perl -MMac::InternetConfig -le 'print +(GetICHelper "mailto")[1]'