I think your cracking in the scratch coat comes from it drying out too quickly and adding cement is the likely cause of that quicker drying out.
The way lime mortar works is through calcification. It absorbs CO2 and basically reverts back to the stone it was made from.
Cement is different. (sorry that is a bit of a cop out but I have pasted some info for your perusal.)
Lime mortar on its own will give you a strong durable finish. Use 3 sand to one Hyadraulic lime (the stuff you mentioned) using 02 grade sand for the finish coat (a grittier sand 04 for the scratch coats).
Damp down the scratch coat well before applying the top coat to help it bond, hopefully it is well scratched.http://www.buildingconservation.com/art
This is a good article and supports the 3 to one mix.http://www.minervaconservation.com/arti
(ps I am not a professional builder , just an enthusiastic amateur who has listened to and learnt from good builders and through trial and error. My renovation is in Brittany and the mixes and finishes bli end in with what's around )
Peoples opinion varies on this.
Only dead fish go with the flowhttp://myrusticview.com/