Here is my experience in cleaning (and bleaching!) a 10-year-old Modernica Nelson Bubble Lamp that had yellowed areas. Note that the manufacturer warns against bleaching. I ignored this advice because the lamp was relatively inexpensive (bought at auction) and I was willing to accept the risk. I tried a multi-stage approach going from mild to severe cleaning until I was satisfied with the result. Try this at your own risk!
Stage 1: Soaking and mild scrubbing with a trisodium phosphate (TSP) solution. This removed some dirt but did not whiten the yellowed areas.
Stage 2: Twenty-four hours wrapped in TSP-saturated towels (see photo 1). Little change.
Stage 3: Repeated applications of "Clorox Clean-Up," a bathroom spray cleaner containing bleach. The yellowed areas lightened but didn't disappear.
Stage 4: Repeated applications of "Clorox Precision Pour Bleach Gel," a toilet bowl cleaner. The yellowed areas lightened further and the shade had a satisfactorily uniform appearance. I rinsed the shade repeatedly and allowed it to air dry.
Although it's a pendant light, I will use it as a floor lamp. The shade is temporarily perched on a Ikea Fillsta lamp tripod (photos 2 and 3) while I await parts for a more permanent stand. The light from the floor lamp is warmer than that of the Nelson Crisscross Saucer pendant above the table, but that might be attributable to the bulb being warmer (lower Kelvin temperature) and of substantially lower wattage. I'll replace the bulb with a higher Kelvin bulb in the future.