With regard to your first question on when two products are used together, I think you're talking about whether it's mixed at the same time and who would be responsible if something were to go wrong. Generally, the label covers what is allowed or not allowed. If a label specifically says that this product is not to be mixed with any other herbicides or whatever, then it would be an infraction of the law to do so. If it's silent on that, then the user needs to check and find out from the registrants if there are any issues with mixing the two products.
In terms of the safety, the assessment that PMRA does is based on the active ingredients and whether that product can pose a safety risk or an environment risk on its own. Any product that has a similar mode of action is also taken into account. For example, when they do their risk assessment for what you're consuming, it does take into account similar modes of action. It may not be the same type of product, but if it acts the same way on your nervous system or there are any potential health issues, the impacts of those other types of products are also taken into account.
It's not a mystery in terms of when products are mixed. It's quite often done for resistance management, for example, to make sure that you're not selecting for a weed that can develop resistance. That's a fairly common thing that the act would easily be covering.
I'll leave the mission statement to whomever provides independent advice to my colleagues here. But in terms of what to do about resistance, in fact there is quite a bit on resistance. It may not be in the act, but it may be more in the regulations. All of the products have a mode of action, a group. For example, herbicides fall into different groups in terms of their mode of action, and that's on the label. A farmer knows. Even though he may be rotating from brand A to brand B, if they both have the same group 2 indication on there, he knows he's not really rotating chemistry, and that's not a good thing. He could be selecting for resistance. That resistance labelling is on all of the products.
In addition, many of the labels will have additional use instructions, for example, “use only once per year”, or “rotate with another different mode of action on an annual basis”. There are some specific resistance prevention measures required on a label for growers.