The solder on that production board is almost certainly lead-free. The problem with mixing the two is that it could cause the new solder mix to be brittle and prone to cracking putting you right back to where you were. It would probably be best to wick away ALL of the older solder from that joint and resolder it with leaded. As for the fan, I would check for a welded contact on a fan control relay. If it was arcing and sparking elsewhere, it could have caused some issues elsewhere. This of course is presuming that the fan ISN"T supposed to be on all of the time.
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