OK, so an update on this: they turned out to be root aphids and the population exploited. Neem oil became uselss as they were mainly on the roots. So I read somewhere to drip the rootball in soapy water for 30 minutes to drown them. I did this and one of the plants, the one that was very affected, seems to be improving and most root aphids seem to have died and are, at least, no longer lurking in the surface, but they still remain on the roots. This plant's roots had darkened, probably due to the damage the root aphids had done.
On the other hand, the other plant, which was much bigger and better in every way went through the same treatment: its root ball was much larger and white, but after the bubble bath, the plant has been wilting for at least 1 week. It seems to recover after sunset and seems mostly affected when the sun is shinnig. It has lost many leafs and might never recover, but at least it is alive. I don't know why it took the same treatment so bad: perhaps its virginal roots were very sensible in comparison to the other one that had already browned out.
I think the soapy water was a bad idea: it did more harm than damage. It sucks, because the affected plant was in super shape before that. I have now bought Pyrethrin, and submerged the root ball of the smaller plant, the one with brown roots that took better the soapy water treatment, into it for 5 minutes and nothing bad happened to it. Haven't checked if root aphids are gone but I have also ordered some Amazing Dr. Zymes, and perhaps will do a root ball bath in it, depending on how persistent the aphids want to be.
Any idea how to help my wilted plant?