I am assuming you have some kind of porch to want to leave the baby at the top and take the chassis first? Most people would leave the pushchair and take the baby first!
The Cruz certainly doesn't feel that light when you are carrying it. It's amazingly long for a compact pushchair - in fact I believe the Vista is longer, and its only something like 1.5kgs lighter than the Vista which is nothing. I am struggling to lift the Cruz into my car - and I am used to throwing large double pushchairs in. The length is part of the issue for me because a smaller folding pushchair eg the Bee does throw in really easily. The Cruz fills the length of my boot. The Cruz also has horrible pointy bits when folded, and I have also trapped my hands twice in the chassis - it gapes open to allow it to stand when folded but thats no good when carrying. Its actually scaring me to use it which I know sounds daft but I honestly dont know where to put my hands once its folded. There is even a part of the chassis that is marked as a carrying grip - but thats where I first got my hand caught. I honestly can not imagine carrying it down any stairs. I would rather carry a double pushchair with a sensible carrying handle / place than a Cruz. The basket is fabulous though!!
The Baby Jogger City Mini is definitely the easy option - simple to fold one handed with a wriggly 12 month old crawling baby under your arm - the carrycot bit is the easy bit!! Think ahead to where you put a non-sitting 7 month old perhaps or as I said a possibly non walking but crawling 14 month old? I really wouldn't worry about the Baby Jogger carrycots - as ihateparties says, there is a choice of 3 carrycots. I had two over 9lbs babies and they were both in carrycots until 6 months old. We also used a Bugaboo Cocoon in our City Mini, and other people have added various soft cots - there are lots of options.
The Baby Jogger Versa would give you a parent facing option, but still with a one handed fold. But the Versa is heavier.
A one handed fold means that you can stand at the top of your steps or get on a bus, and fold the pushchair with your baby in your arms.
The Bee would sling over your shoulder to carry down the stairs. It is fiddly to use - it took me around 3 months before I felt happy with it, and whilst it isn't my favourite pushchair - mainly for the push which is a bit light for me, I think for what you want, it may well be the best pushchair for the job. I have come to appreciate how light the Bee is, and just how easy it is to fold - although it is a 2 handed fold - but if you carry the baby downstairs first, then you can come back for it, in one piece.
Have a look also at the Mutsy Evo. It folds as one piece. Has a good seat, small fold and importantly a carrying handle! Its been one of my favourite pushchairs this year. But it is still a bit chunky when folded. Look also at the iCandy Cherry - it is a 2 piece fold, but if you are having to come back for the pushchair, its easy and really light to carry both pieces down with you. The chassis in fact can be pulled along.
If you give me a few days, I can take some photos of the Cruz next to the BJCM4 and the Bee if thats helpful, but you sound like you have seen them all already?