It is very important for the warm, moist air to be exhausted outside, not into the attic. Duct tape is a lousy choice for ducts, despite it's name. Use the foil tape instead. Yes, you can use a zip tie or large hose clamp to attach the hose to vent. Top it with the foil tape for a good seal.

The mold might be an issue. I'd probably start by putting down some plastic to protect the insulation and spray down the underside of the roof decking with some straight bleach. Don't go crazy, just enough to saturate the surface. Eye and face protection plus old clothing would be a good idea.

I too was a victim of the crappy zip tie from a previous owner that left damp air to pour into my attic. The zip ties are a cheap substitute for a metal hose clamp that save the fan companies a few pennies - and the zip ties fail after a few years. Get the metal hose clamps at your local hardware store.

I'm curious only.. where do you live? I was suprised to hear you have mold in the attic from the amount of moisture from a bathroom.

If you're in Houston or other humid area I can understand
the moisture never leaving.

I would ask about the 'as is' sale of your house. that is,
if it was not 'as is' then you have recourse especially
if the mold is due to the duct not being connected.

Where do you live?

I say I'm surpised to hear of the dramatic effect of the fan from a bathroom into the attic

I say this as in my experience it's usually little moisture
from a bathroom fan and the attic being warmer or vented can get rid of the moisture pretty fast.

Good luck. I'd figure your home purchase is going to
be backed up via the real estate agent who sold the property.

Anyone know where he goes first to complain?

I'll have a look back in a week or two.
Andre

this has happened again band clamps this time!

Use band clamps and a section of rigid duct to extend the connection from the roof, using foil tape before you put on the band clamp. You can also use the variable angle sections of pipe to get it oriented toward your bath fan housings. If you can do the entire run with rigid pipe it will flow much better, but you might need to use a short section of flex pipe to make the final connection (use aluminum flex pipe, not the cheap foil).

What I normally do for a connection that hangs unsupported (like off the roof vent fitting) and may never need to come apart for cleaning is to connect the rigid pipes with foil tape, and then run three self-tapping screws through the flanges on the two pipes, equidistant around. I then run another strip of foil tape over the screws for a final seal.

Bruce