I'm not usually a big fan of repotting, but given the symptoms and the odd potting mix, it might be a good idea to very gently unpot the plant and inspect the roots. If there are some healthy roots remaining, be careful not to damage them and then repot the plant into a much smaller pot filled with a porous, soilless potting mix.
It may be that you do not find any healthy roots. Then it would ordinarily be best to take some stem tip cuttings to root. However, I'm not sure you have any stems with healthy growing tips, so that may not work.