Yes the mulm serves as a source for nutrients also, but in order to be certain that the plants are getting everything they need its important to add fertilizers.
You can use the oil dri by itself, the purpose of the topsoil is that it has a huge supply of nutrients in it, so it negates the need for root tabs. It's also great for low tech setups because it's a kind of set-it and forget it approach.
No I have not charged any substrate, adding ferts to the tank, liquid or dry, will allow it to make its way to the substrate. I could see the point in doing it but you have to be careful not to overdo it, especially with the macros like no3. It'd be easier (to apply and to monitor) just by adding root tabs.
Look at it like this, a clay based substrate is like a nutrient sponge, soaking up nutrients to make them available for the roots. A basic sand or gravel substrate doesn't soak up anything, although in time the biofilm/mulm does a decent job.
So using a proper plant-friendly substrate isn't essential to keeping plants, but IMO it definitely helps.
