Why wouldn't the defence have brought this up, if DM had been leasing the land to someone else, afterall, RP could then once again *attempt* to raise reasonable doubt, by saying, 'hey, see? It *couldn't* have been DM incinerating bodies in The Eliminator, since someone else was in fact leasing that land!'
Speaking in general terms, reasonable doubt is supposed to be reasonable not frivolous or fantastical or merely conceivable.
In order for the defence to introduce a third party suspect into a criminal case the scenario has to have an "air of reality":
In order for the judge to put a defence to the jury, the accused must point to evidence on the record that gives the defence an air of reality (R. v. Cinous, 2002 SCC 29, [2002] 2 S.C.R. 3). The trial judge must determine whether there is some evidence that is “reasonably capable of supporting the inferences required for the defence to succeed” (ibid., at para. 83). The air of reality test applies to all defences, and acts as a threshold to ensure that “fanciful or far-fetched” defences are not put before the trier of fact (para. 84). When applying this test, the trial judge must take the evidence to be true and must not assess credibility or make other findings of fact (para. 54).
https://scc-csc.lexum.com/scc-csc/scc-csc/en/item/14676/index.do