Yes those are immature seed pods in the top picture and yes it will self seed if given a chance. As you seem to have it in a container it wont have the same chance to do so one in the garden would.

It's Euphorbia amygdaloides OK, but I don't reckon it's 'purpurea'. There's nothing red or purple about it.
Mine has red stems under the flowers at this time of year.
http://www.growsonyou.com/photo/slideshow/197370-taken-on-11-4-11/member/beattie

I have a series of photos of three of mine that were eaten down to stumps by the dratted rabbits, so you can see them at different times of the year. They're here -
http://www.growsonyou.com/garden/show_garden_item/26728-euphorbia-amygdaloides

They always have a red/purple look to the foliage and / or stems.

Yes, they don't have red/purple look on the whole.
But under the leaves they are mildly purple and the stems are darker red.
I guess they are in shade/partial sun, so the leaves remain green.
Plant tag says it is 'Purpurea'. May be wrongly tagged.

Plants are frequently wrongly tagged, not necessarily the fault of the GC staff either. Customer pulls out a label, reads and puts back in the wrong pot...

so true.