ICs with infiltrate can not confer infiltrate to units that do not have it. There is no grey area, it simply can not happen. It’s got nothing to do with the Infiltrate rule and whether of not infiltrate confers. The rule says that it does. However, there is no way to attach an IC to a non infiltrating unit which will allow the combined unit to take advantage of the rule.
Imagine this – you have a Farsight Bomb and Shadowsun. You want to infiltrate the Bomb – simple, the infiltrate rule says the rule confers to the unit, so you attach Shadowsun to the Bomb and away you go. Well no… and the answer is no, because of the IC rules, and the deployment sequence.
ICs can only attach to units at the start of the game (i.e. allowing the unit to benefit from infiltrate) in 2 ways – either by being deployed in coherency with the unit, or by being placed in reserve with the unit and declaring to your opponent that’s what you have done. These are the only 2 way it can happen.
The deployment sequence is first normal deployment, then infiltrate, then everything else is in reserve (from which one can deep strike or outflank, but that doesn’t matter). Shadowsun can take advantage of all three deployment options. However, only units with infiltrate can infiltrate (duh!). This is important, because when you come to deploy the Bomb, they do not have infiltrate, because Shadowsun has not yet joined them. She can only join them after they deploy, by being placed in coherency with them.
So when you come to deploy the Bomb you only have 2 choices, either they are deployed on the table, or they are kept in reserve.
If you deploy them on the table, you can attach Shadowsun by placing her in coherency. The problem is that both units are now on the table deployed. You can’t then take them off and infiltrate them. Alternatively you could keep the bomb in reserve, and attach Shadowsun to them in reserve. However, by then both units are in reserve, and the combined unit has missed its chance to infiltrate.
So either way, you can not attach Shadowsun to a Farsight Bomb and infiltrate them.
Nor need people worry about Illic Nightspear, the new Eldar IC infiltrating Wraithguard right in front of your gun line first turn, same problem applies.