I like the idea of the gulf being very shallow and almost unnavigable; that's pretty cool. Your rivers, as Zach pointed out, do some kind of weird stuff -- in particular I'd expect the far left one to drain into that nearby gulf, the way the Colorado River drains to the Gulf of California. (If you wanted to leave the big central capital city on a river, though, maybe you could have a manmade canal through the narrowest point of the continent at the south end of the gulf?) Just keep in mind that if you want a river to go straight, you need to make it go downhill. Something that might be useful to you would be to go over the whole map and mark what you want to be the highest points, connect them with ridge lines, and then put the rivers in afterwards. And then the lakes would go wherever the rivers are on a very slow downhill slope.

Temperate rainforests may be the solution for your biome thing. You can totally have those in the same climate band as boreal forests and just further away from the coast. That and the fact that you want volcanic soil and a steep rain-shadow means that you should maybe be looking at the Cascades as a good model for what to expect -- what do you think?