Now this RX (129K miles) runs beautifully but Toyota told me that it needs to have the Valve cover gasket replaced because it leaks and the differential pinion seal replaced because it leaks.

Will the car be ok without these items replaced? I don't see any drops or puddles at all. And where do I check for the corresponding fluid levels so I know how badly they may be leaking or losing fluid?

Are these major repairs that only Toyota or Lexus can handle? Or can my trusted neighborhood mechanic do it?

 

If that is the case, yes, the leaks should be repaired, it will not get any better, only worse.
Some will add additives for leaks which I personally do not recommend.
Monitor the valve cover leak by checking the oil level on the dipstick. The differential leak is more complicated as there is a filler plug that must be removed & checked by putting a finger inside to feel the level of oil.
I had a differential leak when I purchased the vehicle, dripping from the seal. The part isn't expensive (around $20), the labor depends on where it is done. My trusted mechanic charged me $50.
The valve cover leak, hopefully some other CL members can help.

 

My 2004 RX330, with 150K miles on it, has the same problem - valve cover gasket oil leak. It does not drip on the garage ground, but I need to check the oil level and add more around every 1K miles; but I can see the oil leaks from valve cover all the way to the bottom plastic cover ( as doing oil change ). And the mechanic told me it costs $1300 to fix. I don't know if it's worth it.