If they infected the geneseed would that work?
Currently there is no way to do that, and if they did (in some fashion as yet to be seen) the seed would be discarded as defective. Basically, it won't work.
would the points in the Genestealers cult dex reflect the points in the IG dex?
As the stats are pretty much the same I think it could work out.
could the cult get their hands on sisters of battle/demon hunters/squat vehicles? if not, why not?
It is possible, as these rely on imperial guards and PDF (who you can infiltrate) for their backbone support, while the marines maintains their own, meaning they are a lot harder to get to.
Could the cult include a select few tyrinad organisms, that they brought down in a spore mine and hid in the HQ?
Not according to the fluff of the cult. They were the vanguard, and meant to be as inconspicuous as possible; grab some civilians with the purestrain genestealers, start up the cult, one purestrain turns or breeds up a Patriarch, you get hybrids, and build up a force of new-gen purestrains, and then you strike.
could they infiltrate deamon worlds or chaos controlled worlds(minus thousand sons)?
I doubt it. They would probably just spot the infection psychically and do away with it, or corrupt it. That used to happen, and there was a Genestealer Coven-list for it, with chaos-mutated hybrids, beastmen and patriarch with daemonsword. Fun times.
If for whatever reason the pyshchic connection to the hive mind was broken what would happen to the cult?
They go rogue. It happens in the fluff too, and then the Cult will actively fight the nids when they arrive, having claimed the world for their own, and either get overrun and eaten as anything else, or pushing them back to take on something else.
Are orks infiltrateable? again if not why not?
They used to be, when they bred sexually, there were even ork hybrid models. Now that they are spores I cannot for the life of me see how it could be even remotely possible.
In essence, the Cult can only work on Humans, Squats (and other abhumans) and Tau. Note that marines, daemonhunters and other non-breeding “humans” are not considered humans.