After some struggles with the high L/D ratio EDPM liner I managed to find an approach which worked well both inserting the liner into the casing as wells as inserting the grains into the liner. Below sequence seemed to work best:

1. Talc both the casing and liner (in & out), remove excess talc.
2. Insert both Full Length Coupler Tubes (FLCT) into liner. Measure liner length: 1629mm.
3. Insert liner and FLTC assembly into casing, overlap on the side with loose flap facing bottom.
4. Remove FLCT’s.
5. Dry fit nozzle (no o-rings) and secure retainer to position liner into casing. Press liner against  nozzle.
6. Insert grains one by one (except last one) into casing from top. Use PVC dowel to push the grains in. Use a rolled A4 office paper as a sleeve to insert the grains in the liner.
7. Remove nozzle, grease new o-rings and nozzle. Clean casing nozzle end from talc, add RTV inside casing + top convergent nozzle and grease. Insert nozzle into casing and secure retainer.
8. Press liner and grains from top into RTV bead.
9. Measure clearance liner to forward end casing and thereby verifying the liner is butt up against the nozzle.
10. Grease new o-rings and forward bulkhead. Clean casing forward end, add RTV and grease.
11. Place motor vertical and insert forward bulkhead and secure.