chestnut compound works. copper sulphate/copper carbonate. it doesnt penetrate the plant much but it will add something to the taste. a good micronutrient feed will strenghen the poor thing. stop picking leaves, but get rid of those dead ones.
what i do if i think a leaf is going to fall is give it a little tap. if its dead it will fall off, if not just leave it.
check the roots. it looks like the plant may have been getting overwatered a bit. watering is a bit of an art IMO. i usually wait till the compost is just shrinking from the sides of the pot (couple of mm) then water well.
if the roots arent white then that would explain everything