Regarding the poison immunity, the wiki page says: "[Anti-Poison Skill] is theorized to stem from their contract with Mash Kyrielight, bearing a high poison resistance or blessing skill, neutralizing any Filth-type threat beneath the Mystic level of a Noble Phantasm", which seems to strongly indicate that it's from contract. So we're afraid unless there's concrete proof that she'd keep it even when her pact is broken with Mash, we can't allow you to regain it as that'd be regaining another person's power.

As for clarification, this would mean choosing a specific servant! You would have to select a specific shadow servant to regain.

And as for basic attacks, we're talking about they can swing a sword at a normal human fighting ability levels. Something that you are describing would require a lot more points as it'd count as an additional ability.

I hope these clarifications help!