The most important factor when trying to take the "one bag does it all, I'm gonna shift the down" is how well constructed the bag is. Here's the thing. I've got a WM Antelope DL that was my only bag for years. It's awesome! (Also a bit heavy at 2# 13oz, but it's a 5 degree bag, too.) The reason I say construction is so important is that many, many bags on the market don't start out with enough down in them. If there isn't enough down in the baffled chamber to start with, (1) you'll have difficulty eliminating cold spots and getting even loft at any time, and (2) when there's less down in the chamber, it tends to shift more on it's own accord and without your explicit blessings and encouragement.
I've spent far too much of my life on the retail end of this industry, and from the non-mail-order companies, Western Mountaineering and Feathered Friends are consistently on top of fill levels. Many other companies skimp on the down.
As far as practicality is concerned... Shifting the down is very easy. Open the zipper full length. Grab the bag at the zipper on the side you want to move down out (i.e., grab the top of the bag side zipper to shift down to the bottom). Shake vigorously. Check the bag visually, and feel loft with your hands. Adjust as necessary. Very easy process. But not one you're ever likely to do during the night. Shifting down is something you'd do either pre-trip or before bed. Incidentally, if you want to get the bag back to its original temperature rating for "factory" loft, most companies rate their bags with 60% of the down on top and 40% on bottom.
As long as you sleep flat on your back, this is eminently practical (not particularly from the perspective of weight, but of versatility in temperature range and ultimate cost). If you tend to sleep on your side, you generally should roll with your bag so that you aren't exhaling all your humidified air into the bag and reducing your loft. Bottom line, I wouldn't plan on shifting all the down to the bottom, and if you did, I'd plan on spending at least part of the night on your back when you get cold.
I know that a whole lot of people on this site would disagree with my following statement, but here it is: I'd probably suggest a zero degree bag for ultimate versatility, given that this is a system you want to use in winter. Generally you can add 10 degrees to a manufacturer's rating to get to a warm n' comfortable range. If you're out in summer, shift a bunch of down to the bottom. No big deal. It really depends on how much of your time you'll spend doing what. If you'll spend 80-90% of your time in 3-season use, just get a 20 degree and layer up in winter.
Thinking back to your original post, MAYBE the ultralight option would be best. What I'm thinking, honestly, is "Holy $%^#! Six layers on the bed?" If you need that much fine tuning at home, get a large-cut 30 degree mummy (Feathered Friends Kestrel, perhaps Osprey--Grouse is the women's cut, not sure if it'd be big enough-- or Western Mountaineering MegaLite, maybe their semi-rectangular MityLite-40 d- or Sycamore-25 d-) , and in that large-cut, cool-ish bag wear a bunch of layers you can strip off at will, definitely including a down jacket, perhaps preferably with hood.
Anyhoozit, that's my couple cents