When buying varnish, as you've learned, it goes bad, always buy fresh, or test it first, if it doesn't cure correctly, take it back. I don't know what thinners/solvents Canada allows, but there are excellent varnishes that are thinned with only mineral spirits and maybe naptha. Normally, theres no need for any of the more unfriendly solvents in varnishes.

When you buy varnish for a wiping mixture, like you are using, make sure it is thinned with petroleum thinners, that way you will be assured they are compatible with linseed oil and mineral spirits. Not all modern varnishes are thinned the same, use the wrong varnish and you probably won't like the results. As an aside, turpentine is an excellent thinner for any oil based product. for some applications there is probably no benefit, but for example, thinning varnish to brush, it is much better than petroleum thinners.

Tim