G'day Graham,

That's a personal preference generally, it comes down to your sleep system & the ambient temperature on the trips you are doing.
You also have the option of going with a quilt aswell.

G'day Jared,

It doesnt, sorry - the being part of the Spark ultralight bag range, the zip functionality is minimal.  It has one zip running roughly 1/3 of the way down the bag.

G'day Jack,

There isn't an exact answer and it is a bit of an estimate for using the Spark SP0 inside the Spark SP3.

Together there is about 530gsm of down insulation, so it would put you between the SP3 and SP4 comfort ratings of about −4 °C to −6 °C. 

G'day Suzanne,

You could probably make it work if you're desperate, but its not a great match up.  The Spark Male/Unisex bag is quite a different size and shape to the womens specific Flame series. 
Given the investment, we'd recommend you go with the Flame Fm0 instead.

G'day Jack,

The SP0 can be used as a liner in the SP3 or SP4, as they are 'expedition width' bags to allow for an extra liner or clothes to be warn inside them. The SP2 is the same size as the SP0, so while you can make it fit if you're on the smaller side, it won't allow the bag to fully loft.

G'day Jared,

It wont, but compared to other quilt options the Ember has comprehensive mat straps.  These are obviously designed to wrap the quilt around a sleeping pad, however, they also allow you to cocoon the quilt around your existing sleeping bag as an alternative.

G'day Paul,

The Spark SP0 has a comfort temperature of 10C, so when used as a liner it should add about the same to another bag. Based on the construction and materials if it has space to loft then it should add more warmth than the Thermolite Reactor Extreme.