Cooking & Baking

The also get soft as they absorb extra moisture from the air.
Onions that go bad smell "off", because the five primary taste/odor components are sulfur containing organic compounds.
When they go bad, these can oxidize or be hydrolyzed into propyl sulfide (rotten egg like smell) of propyl sulfoxide. The latter has a pungent, acrid odor much stronger than plain onion smell.
If they still smell like onions, they're almost certainly OK.
The translucence you describe is probably excess water, causes the volume to swell a bit. Given the same amount of fibrous tissue is there, making them swell with water will reduce opacity.
Yes, I have found that peeling an older onion is more difficult as the thioethers tend to get sticky when the moisture content gets high enough. I'm guessing you noticed that outer layer was a little slimy compared to a fresher onion. If so, there's your reason. You said they were sticking to your fingers.
Isn't food chemistry a hoot?