Hi Julie,
The most important first step you can take is to find the best and most knowledgeable manufacturers or outlets and only then start thinking about the mattress. They will have the expertise and knowledge that it would take you years to learn and if they are “on your side” … then you don’t have to learn anything expect the basics (reading the overviews in the mattresses section will help you ask better questions and better understand the answers).
When you have choices between 2 or 3 better outlets … then between them and other online outlets for reference purposes it’s fairly easy to make value comparisons and choose between them. You would then be choosing between “good and good”.
The more they know and are “on your side” … the less you have to learn and this is by far the simplest way to buy a mattress. In person testing with the help of an “expert” is far more accurate than going by your personal specs or the specs of a mattress or wondering about “theoretical” questions that only your personal experience can really answer.
Blended Talalay is just as safe and has the same certification as natural talalay latex. they are both very safe materials and softer blended talalay has the advantage of being less costly and more durable than the softer natural talalay.
This is too general and subjective a question to really be answered with any accuracy because the only accurate answer is “it depends”. Only your own experience can really answer it in other words. A 2" topper though … depending on the ILD … will certainly make a mattress softer. Whether that was enough would depend on many factors including the ILD of the topper and what the makeup of the mattress under it but mostly how far away from your ideal the base mattress was. these types of uncertainties and trying to answer these types of questions based on “theory at a distance” will lead to more frustration and confusion though. It’s always best to find the local people that can best help you and then they can see how you respond to various mattresses and layering in real time and with their own eyes.
You have some quite a few good choices in the Vancouver area besides the ones you mentioned and many of the better ones are listed in post #2 here.
If I was in your shoes … I would learn the basics and then forget the “specs” and focus on your personal testing and on what your body tells you with the help of someone who has your best interests at heart. Phone all the outlets first to talk with them and ask them questions and then pay the ones that you best connect with a visit. You will bypass most of the frustration and uncertainty that way 
Phoenix