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This article should answer your questions.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-and-calcium.htm

A general purpose fertilizer designed for vegetables should work fine for your vegetable garden, provided that you are meeting the plants other requirements (sun, proper soil pH, water, temperature, proper drainage, etc.). Nitrogen doesn't stay in the soil as long as the other main elements (phosphorus and potash), so you'll want a fertilizer that has a decent amount of it (6 percent could work). I don't know where you live, but, here in Virginia, a soil sample test sent to Virginia Tech's Soil Test Labs costs $10 per sample (for Virginia residents) or $16 (for out-of-state residents). That's not terribly expensive if you plan to grow a lot of vegetables.

I would follow the instructions. Although I do not believe that it would cause any harm, I do not think that it would utilize the full benefit of the fertilizer.

I do not know the reason that greensand is not readily available in Canada. I do know that I answer this same question quite often, so it is difficult!
I did locate a possible source for you . Welcome Harvest Farm LTD 1604-486-7619 This company is in Canada.

You will have to be careful with this. Usually nitrogen is in the slow release formula, so make sure that you are aware of the NPK ratio and how much your plants can handle. You will need to adjust accordingly with the phophorus and potassium.

This pdf is an extraction method for rice hulls: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwj9xPHXrq7aAhWr7YMKHSgQDSsQFgg2MAI&url=http%3A%2F%2Fwww.scielo.br%2Fscielo.php%3Fscript%3Dsci_arttext%26pid%3DS0100-40422011000100014&usg=AOvVaw3gxgZb2PinYw4LVTzuaRt4