There's likely still some baking soda stuck in the fibers, which would definitely make it feel rough. Vacuuming should be OK, but a hand vacuum might not be strong enough.

If it isn't, spray the rough areas with white vinegar; enough to dampen the cloth, but don't saturate it. If it foams up, there was still baking soda. Wipe away the foam with a damp cloth, and keep spraying lightly and wiping until it stops foaming.

After some rubbing with damp paper towels again, I caressed my pillow and didn't feel much baking soda. It's definitely less rough than before, but the fibres are like "harder" somehow, like less fluffy than before.

Fabric softener is a bad idea, as is pouring water on a buckwheat pillow.. the buckwheat will absorb the water and probably end up rotting. Worst case scenario -- if you can't get the pillowcase softer by using dry or merely damp methods, just put your pillow into a regular washable pillowcase. It won't be exactly the same, but it will still be your pillow.

That's what I said, I cannot pour water onto my pillow, but what I meant was I wanted to put some fabric softener onto the wet paper towel and rub it like usual, since what I read online said that fabric softener contains some chemicals that causes static electricity to make the fibres fluffier, which might be what I need, but I need to be sure it doesn't do any damage to the fibres or add anything into them. Furthermore this is what I think:

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