I'll do the tl;dr version of Cuip Draw.

1. For the Cup draw the Top 14 players by Q are used.

2. Any player in your Squad whether signed or unsigned can count towards the calculation if they are in the Top 14.

For a team with 14 high players all 3* or higher the replacements for OOC players will have no impact on the draw.
For a 14 player Squad with 3 low Q players, manipulating the Cup draw, any replacements for OOC players will impact on your Cup draw as any replacements will be higher up in the Top 14 than the lower Q player(s).
For a squad with 14 players which include retiring players the replacements will lower the Top 14 Q for the draw.