If one prepares soda carb in an oven from bicarb, wil the soda carb eventually turn back to bicarb? Also, is soda carb only able to basify acetate salts or is it strong enough to do entire exatractions with?

no it wont turn back to bicarb, it's not a reversible reaction.

Sodium carbonate is strong enough to do extractions with but it has it's issues. Here are two examples of sodium carbonate-based methods:

https://www.dmt-nexus.me...aspx?g=posts&t=44863
https://www.dmt-nexus.me...aspx?g=posts&t=30501

If you want to do a normal wet A/B, then you gotta remember to add the sodium carb very slowly otherwise full volcano effect and spillage possible when sodium carb starts neutralizing the acid. Also more chances of emulsions to occur so its recommended to add salt. Also if you use a very selective solvent like naphtha, yields might not be so good due to the pH not being so high, so either add a less selective solvent or maybe use calcium hydroxide in conjunction with sodium carbonate so as to create sodium hydroxide by the mix and therefore increase the pH and yields.

I also had a question about SC, hope I can leave it here (it doesn't contradict the title )

My (washing) soda's ingredients list sodium carbonate 30% as the only ingredient.
They're 'moist'/ 'wet' crystals/ lumps.

So my question would be- is S.C. hygroscopic and is the remaining 70% just water that I can remove by crushing the lumps of carbonate in a mortar and dry in the sun or in an oven to get pure(-r) S.C.?

Find msds for product and/or email company to ask if it's indeed just sodium carbonate and water. If it is, then you're good to go, yeah you can just dry it with heat.

Otherwise just go buy some sodium bicarbonate in the nearest pharmacy or grocery store or supermarket and make sodium carbonate from it by heating.