If only partially toning - yes.. you really should fix again - since there will be some un-redeveloped silver halide in the emulsion. Thiox toning is nothing if not a redeveloper...! I've been doing alot of thiourea dioxide toning lately (which, I believe is nearly identical chemically) in preparation for toning some 40" prints (that I'm quite nervous about!) - so I've been getting alot of experience and doing a fair amount of research into. The acetic acid, I'd surmise, would work - though I've never had any problems like that at all among the fifty or so prints I've toned thus far. LA tapwater is super hard too...!
As for the spotting thing - well - I'd be pretty cautious about spotting at that point - as the farmer's reducer may affect the thiourea development in that place. Personally - I'd spot-bleach it AFTER toning - and then spot with appropriate colors to suit. Just my 2 cents.