... out of fear of it becoming a lightning rod...
How much of a problem this is will depend on a lot of factors, especially how prevalent
lightning strikes are in your area and how you handle the antenna feedline.
But often the
fear is the problem rather than the actual risk...
... just some duct work that sits about 1 foot above the ceiling joists...
This will, of course, affect the antenna resonance and performance, as will any other
ducting, wiring, metal flashing, etc. You can't just cut the wire to formula length and
expect a low SWR. (That doesn't always work outdoors either.)
Interference to (and from) electronic systems in the house can always be an issue -
you'll have to try it and see. Sometimes it can be made to work - there are plenty
of hams operating with attic antennas. Sometimes it may require filtering of alarm
system wires or other problem areas, or turning down the power. Sometimes it just
becomes too much of a nightmare.
My general approach would be to use a dipole with the feed point in the center of the
roof, running the ends as far as possible along the longest diagonal, then bending them
around the corner and along the side to use up more wire.
There are two general approaches: multiple dipoles on a common feedpoint, with each
tuned for a specific band, or a single wire fed with twinlead or ladder line to a tuner.
(Many multi-band antennas such as G5RV, trap dipoles OCFD, etc. may be difficult
to tune to account for the nearby metalwork, wiring, etc.)
From the perspective of minimizing the number of trips into the attic, the "doublet"
as long as you can reasonably make it, fed with twinlead, doesn't require any tuning
or adjustment of the wires once it is in place. I've used TV twinlead and dropped it
down through a slit in the ceiling right to a tuner beside my rig. That approach also
allows operating all the bands between 40m and 10m (though efficiency might not be
particularly good on 40m.)
You can improve 40m performance by extending the wires around the corners and/or
adding loading coils (I'd probably do both). But start with the simpler installation and
see how it works.
Multiple dipoles have the advantage of direct coax feed (a 1 : 1 current balun is a
good idea.) I'd start with one for 20m, which should just about fit. You'll have to
make a couple trips up and down to tune it for lowest SWR, since the formula length
probably won't be quite right. See how that works, and if it is promising then add
another wire for 15m to the same feedpoint as well as one along the other long
diagonal with the ends bent (and possibly loading coils) for 40m.) While this method
works pretty well for the pre-WARC bands, it can get messy when you try to add
12m and 17m because the resonances are close together.
Whichever approach you use, provide sufficient insulation at the ends. I'd keep the
wires spaced at least a foot away from any metal (preferably more) and use a good
insulator (or at least a foot of synthetic string) at each end or corner. Light flexible
wire will be easier to work with in a confined space than #12 CopperWeld.
