Yes, great questions and I know exactly what you are asking about.
I was taught this not by a Euro climber but by a Canadian ACMG Guide.

The idea here is that you choose your bag: choose a down bag for max compression as this could end up being a bulky system.
Then you choose your over-bag:
See
The Emperor Penguin Windstopper overbag: Product Number: 5009-397
MEC Penguin Overbag +15C: Product Number: 5003-642

Then you choose your liner.
In the case of the liner, I personally would only use the liner as a way to protect your bag from sweat and grime vs as an insulating layer. The only option for insulation in a liner is often fleece (see: Product Number: 4018-408) and these are bulky.
Your other option in super extreme cold conditions is a vapour barrier liner. This is designed to literally contain all warmth by trapping body “vapour”. See Product Number: 5010-753.

The sleeping bag you choose is up to you. If you choose one that is moderately rated then you can use it on it’s own in mild conditions too making for a more versatile system. However if this is strictly for winter camping and winter conditions then go for a warm bag and then add on or remove when necessary.
See: Down Mummy bags and/or Western Mountaineering bags…under the sleeping bag search category on our website.

The overbag is up to you, read the notes on each and then decide what suits you best.
Some folks also then add on a bivy bag but that is ideal only if you omit a tent (the real purpose of it) as that gets really heavy.

Call us at the toll free Service centre if you want to talk this through some more and need help deciding what your best options are.

The Pharaoh uses a YKK no. 8 2-way separating zipper. The zipper lengths will vary depending on the size of the bag and they can be zipped together with other MEC bags.

When measured according to ASTM standards, the compressed volume under 1 psi of pressure is correct as stated in the comparison chart.

This is not a measure of absolute compression. The difference is that the Pharaoh (down) can be compressed beyond the measurement given in the chart. Once the Hybrid (synthetic) reaches the point at which it can no longer be compressed, the Pharaoh will continue to shrink.

There are several advantages to down such as compressibility, warmth to weight ratio and durability(longevity) to name a few.

Yes the Pharaoh -12C bag will fit into a 10 L Outdoor Research AirPurge DryComp Sack, but just barely. The fit is very tight and requires a bit of work.