Hi Dave, I pack them separately as the bivvy can get wet and perhaps a bit muddy. With the Inov8, sleeping bag goes in bottom of pack - it's handy to have a zipped bottom compartment as it's easier to whip out to air on a footstop - with current pack I have to get everything out...
Usually outside - but rain can puddle on the Ridgerest and a few drainage holes help - to help save wear on base of bag. If already raining perhaps better inside, but it's a devil to move about and stay on it...
I hunker down and make sure it sticks out more at top - this way the bag is better ventilated (they don't breathe as well when wet) My bags are quite basic, but some have a loop with some way of helping it stay open - otherwise you don't get much sleep!
Last year, of course, I just used the tarp and midge tent, but it was like putting a tent up anyway as I decided to take the poles. Just as well as on most nights there was nowhere to rig a line... Interestingly, the tarp was wet with condensation after the 1st night and was wet for the rest of the trip (thus adding to the weight...) even with better air flow.
The brolly might come in handy for head-end cover - I've tried it on the back lawn. I tucked the brolly handle into my arm pit and it worked ok - of course a sheltered site would be necessary! I did try putting the brolly up inside the bag, which gave good room, but a slightly bigger bag would be needed to accomodate - as it was I broke mine, but a stronger umbrella might work. But a small head-end tarp would be good - you can get them but they are heavier than they need to be I think.