As stated, a pinhole leak in the exhaust would produce fumes in the cabin heater. Given all the irregularities with the pilot and charter, it is quite likely that the aircraft was not properly maintained. Carbon monoxide is highly toxic, but odourless, so a special detector is needed.
A lot of people allegedly benefited from the Prasa tenders: https://www.dailymaverick.co.za/article/2019-08-18-gravy-trains-always-on-time-cash-from-prasas-r3-5bn-locomotives-deal-also-flowed-to-jacob-zuma-foundation/
Unusual source, but some very interesting thoughts on the B737 MAX and MCAS design. Also contrasts the safety role of aeronautical engineers, as opposed to the computer programmers who seemed to be let loose on the software...
More details and allegations: https://www.news24.com/SouthAfrica/News/tobacco-wars-hits-threats-and-secret-ops-behind-the-simon-rudland-attempted-hit-20190817
P.S. Should probably be in the News forum.