Auditors are most certainly not allowed to invest in the firms on which they render opinions. Most accounting firms circulate lists of clients in which partners and staff are prohibited from investing or holding other potential conflicts of interest.
The Professional Engineers Code of Ethics prohibits "potential" conflicts of interest. Investment in a client for whom a firm renders an opinion most certainly presents a potential conflict.
I'm sure that you are aware of the evidence supporting the potential conflicts of interest on the part of IsleChem. The primary evidence was a quote of Haphazard John by a JBIIE supporter. That quote is at least as reliable as a great deal of evidence presented herein.
Although not entirely on point to the topic at hand, my position is that Haphazard John was the first to refer to himself as honest in the Toronto Star article before a local newspaper came up with the honest John account. It's interesting that the bank was so impressed with John's self-described honesty that they closed all of his accounts. So yes, the "honest John" story was the product of Haphazard's imagination and was later picked up by the local human interest column. If this story is supposed to cement Haphazard's reputation for having integrity, it's going to be a tough sale.