Thanks Cook,
Glad you like them.
You make nice stuff also.
The sprue does not snap off & even if it ever did just superglue it back. I used plastic bases for this sort of thing so they're stronger. Mdf might come off, coins are not good.I sometimes do integral bases also.
Like on these Barabou Cattle.


Or this dino

As you say it is much easier to mould a part of the figure then sculpt the rest.This gives you more variation unless the figure is flat as are those in pics you showed.
Having said that it is possible to do a 2 part mould with the blue stuff also
There is a huge difference in the different moulding processes. I prefer the blue stuff when doing putty minis as it is more rigid & the part comes out easily enough once dried. If the part is flat & has straight edges & not too detailed , say a backpack you can also make a press mould out of sprue.
If you are casting in resin & the part is complicated then you will have to use a softer rubber.This will allow you to Cast a whole figure even if it has undercuts.
As always there is no substitute for experimentation & trying things yourself in order to learn and you have a knack for sculpting which is the most difficult part.
Hope this helps
Z