Is there a Polish law against appointing trusted people to key posts?
If, as was the case with Misiewicz, a person does not have the minimum professional qualifications required by law to be appointed to the board of a state-owned company, yes, it is illegal to appoint them to that board.
If, as was the case with Misiewicz, a person does not have the minimum education level required by law to be appointed to the board of a state-owned company, yes, it is illegal to appoint them to that board.
If, as was the case with Goss (who 'lent' Chairman Kaczynski 200,000zl), a person already sits on the board of a state-owned company, yes, it is illegal to appoint them to sit on the board of a second state-owned company. And to that of a third state-owned company. And to that of a fourth state-owned company. And to that of a fifth state-owned company.
If, as was the case with Szota, a person does not have the experience needed to be appointed to the board of a state-owned company, for example they are appointed as the deputy director of the research department of Poland's largest gas company on the basis that they have experience owning a kebab stand, no, it is not illegal to appoint them but it damn well should be.
There is one against tender fixing and that was a part and parcel of the PO regime's standard MO.
If you have evidence that any tenders were fixed during the previous governments, I would urge you to contact the public prosecutor so a formal investigation can be started and the guilty held to account.