2. Other planes seem to have been getting in just fine. If the clouds were a factor, they wouldn't be doing visuals. People would be complaining, and they'd switch to the ILS.
After several decades of flying in and out of the US, I can testify that that is not always the case. For whatever reason, quite a lot of pilots seem to want to declare that they’re visual, even though they can’t see the airport, the ground or even more than 100 yards in front of them. Maybe it’s peer pressure, or it means they can do their own thing, but I hear it used a lot in definite IMC.
As far as following aircraft at night, you can do it, but assessing distance is more difficult than during the day; yes you can use the TCAS display but in a high-density environment with poor azimuth resolution, it’s easy to pick the wrong target, and it’s not exactly an SOP either (definitely not for LH!)
Being an occasional visitor to SFO I can see both sides but if an aircraft is unable to do a visual procedure, for whatever reason, then ATC should give them an instrument approach, barring unserviceability, as they are on an IFR flight plan.