Is there anything to stop insiders/large investors buying up the...

Is there anything to stop insiders/large investors buying up the stock while it's still listed and insanely discounted?

If it's taken private, I guess they will be the only ones that could arrange to have their stake in the company valued and potentially sell their stake.

How frequently do insiders or >5% investors have to disclose their purchases?

When it's taken private, how would a shareholders stake be calculated? Is the entire "value" of the company at the time of delisting its market cap?

E.g. if someone owned 10% of issued shares would they own 10% of the company?

Seems small holders get royally abused whichever way it unfolds, and there's nothing we can do about it.