Warren, I think your argument is extremely faulty. Here's why:
First, we've already seen a country with large neighbours that have marijuana illegal in almost all circumstances pretty much legalize it: the Netherlands. Yes, on the books its is currently decriminalized, but in practice it is "legalized" - the state taxes and controls it by licensing cafes and so on to sell it, which is pretty much what Trudeau wants to do. I don't see the doom and gloom you're predicting here happening over at the Netherlands. The same example can be given for some Australian states that decriminalized, and we'll see what happens with Colorado and Washington.
Second, while I don't doubt the zeal of US and Conservative politicians, I don't think the issue is controversial enough for the mass public, or US customs, to really care about. Public polling already has come out and said that most Canadians prefer outright legalization, and a hefty number of Americans as well are supportive; moreover, if the US government decides to start a trade war with Canada over marijuana, then we have a lot more problems than we think with our neighbours to the south. It is such a small issue, especially considering that we're going to regulate and tax it, that its almost as stupid as the arguments that said same-sex marriage would disrupt our trade relations with the US; yeah, some Americans in power may not like it, but do you know what they like more? Money, and they get a lot of it from cross-border trade. Why would they affect that over marijuana? This isn't the freggin' 1950's.
Third, Trudeau's argument just makes so much goddamn sense that if you can't convince someone to agree to it, then I have no idea how you made a living as a lawyer. The idea is simple: legalize marijuana so we no longer persecute a recreational drug that probably millions upon millions of Canadians use, has proven to be less harmful than alcohol, and can be easily regulated and taxed for the benefit of the government. We'd probably follow a similar model to the Dutch, where we license shops to sell it and keep it off the streets. No one is suggesting we allow people to blow smoke in your kid's face or something, the idea is that we treat it like we do cigarettes or alcohol (at least here in Ontario). Its so reasonable that its almost impossible to agree, especially given that it has such strong moral arguments behind legalizing it as well. Not to mention all the tourist dollars we would grab!
Sorry but your arguments are weak, not to mention essentially a big slippery slope fallacy that most marijuana prohibitionists use, except on a different level. I don't even smoke, nor have any intention to, and I know how silly keeping marijuana illegal is. We have a chance in Trudeau to rectify this situation where people are thrown in jail for possessing small amounts by this Conservative government! We have a chance to finally get a politician in there that is up to speed with the rest of the population on this issue, something that probably took a hundred years to do. Now you're suggested that we're moving too fast and we need to slow down? Not likely.
The poll I mentioned, or the one I could find quickly, was this one as noted by NatPo. 40% of respondents said it would be taxed and legalized, with a further 26% saying it should be decriminalized for small amounts. 31% combined prefer the status quo, with 11% saying the laws should be stricter. This is pretty consistent across all regions.
The fact is that marijuana, like it or not, is going to be decriminalized and very probably legalized within the next couple of decades. Popular opinion is just too strong to fight against, because everyone knows the lies said about marijuana's effects and harm. Why would we not get a head start now? Trudeau's position is the right one, and I hope he sticks by it.