I really feel sorry for your car :) Neglected and abused! I am glad the spark plugs came out OK!
Oil under the cover? Normal for neglected cars (I love to dig it in, bare with me). That oil is from the oil cap gasket, which needs to be replaced with something softer, like a new one - cheap, even from the dealer. But do not replace it yet! Not until after you clean out the flame trap and the small hose that goes to it, otherwise, you will create too much crankcase pressure and the rear main seal may pop.

Yellow residue is not good, if it came from coolant. Monitor that closely! Put a grease pencil mark on the overflow bottle when the car is cold and check the level once a week for a while. That could be an indication of a head gasket leak. Minor so far, but not cheap to "fix".

Klaus
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1975 164 w/174,800mi (Sold) 1995 850T w/91,000mi, 1998 V70R w/129,000mi