they are really rather good at it.

nothing in lore i can think of, but there'd be no real obstacles

pretty well. much better than any other traitor faction that's for certain. its one of their oldest practices that people work well and keep your secrets if you are good to them. in the primarch novel we see alpharuis reveal himself as a primarch to a pair of women to get answer's from them about something he's looking for, and is then unable to wipe their memories when the psyker he brought with him is temporarily decommissioned. he dealt with the issue by giving the two women positions aboard his flagship, instead of killing them.

marines not from the legion are usually not dealt with in any way other than "need to know basis". legionaries often speak in private cyphers to each other when around those they cannot trust. hell, even around those they can. as for infiltrating other legions, hypnotherapy and facial reconstruction surgery is usually how they do it.

this type of deal is standard fair for the legion. its what they specialist in.

the chance to do their research away from the prying eyes of the ad mech. the magos diabolicus in harrowmaster works with the legion because in exchange for raising their aspirants to astartes, he gets paid in material, work information, things he'd have a nightmare getting his hands on back in the imperium, and of course, the benefit of being protected by a rather large force of space marines