I have limited space and the most common solutions are not available to
me. From
a practical perspective it would seem to me that building a 40 foot
center feed
loaded dipole and putting it in the attic or on the roof would probably
perform
somewhat better. ..............

Probably so for short/med haul. Will be a toss for long haul.
Even a semi lossy vertical can outdo a low dipole to dx if the
path is long enough. But for talking 200 miles away, the dipole
will blow the vertical away.
12 radials is not many for a ground mount. If you were to add
more radials or screen, I'd do it at the base of the antenna.
IE: add 50 10 ft radials for 500 ft of wire... Should be better
than a few full size radials. Some say screen can cause problems
after a while do to corrosion. I'd probably just stick with wire
radials myself... How far you want to talk on that band should
determine what antenna is best. For most general use within
1000 miles, I'd rather be on a dipole than a vertical.
But dx, I'd go vertical, and improve it. Loaded does not have
to mean crippled. But loading/short verticals do make the ground
system more critical. My mobile is better than my home dipole
late at night on 40m if I'm talking over 1000 miles away...
And if it ain't loaded, I dunno what is...What power it radiates
is at the perfect fairly low angle for that path/time of night.
BTW, I'd install the dipole outside if at all possible. Less noise
from the house, and will generally work better. Won't hurt it to
be close to the roof, unless metal is underneath to detune.
I've layed dipoles directly on composition shingles with no
problems at all. Or at least when dry anyway... But elevated a
bit would be better.
MK