but a little confused as to how to store the camera with lens attached with the provided insert.
You don't.
The 75-300 usually lays on top of the 135 & 100mm.
Under the 350D there is an 18-55 kit lens, memory cards, and spare batteries.
Under the 10s there is film, microfiber cloths, a small flashlight, and a Gerber multi-tool.
Straps lay on top of everything when I close it.
The 50 & 85 fit in the same pouch.
EDIT
This is a great bag - I love it, and I'm sure you will too. Not being able to fit a body with lens attached is the ONLY downside for me.
Still..., keeping the purpose of the bag in mind (designed for photo-journalists, if memory serves corectly) - if you have a lens attached, the camera is around your neck.
Very easy to access while wearing it, no need to set it down to get stuff out (though, obviously, it's easier if you do set it down). The size is deceiving - it can fit way more than it looks like it can. Just look at mine, lol - 7 lenses, 2 bodies, 4 filters, and a bunch of other crap.