I have no idea. Having never thought of using the setting, clearly not me and clearly not you either! Just offhand, I can't think of any situation I have ever come across, but someone somewhere might use it all the time. Perhaps if so they'll tell us.

Try the sensor dust removal several times. Then a rocket blower. If still no success, then a cleaning kit may be needed. Others will follow with their recommendations. I've never had to use a cleaning kit, so can't help with that bit.

I'm going to guess that the Lo rate is the fastest possible rate that can be continuous until the card is full.

The fast rate can of course be reduced by using a slower shutter speed. You can't do 7fps with a shutter speed of 1/2 second for example.

Scuffing and scratching whilst being carried in pocket, briefcase or camera bag I would think.

I suppose what you are asking for is a continuously variable setting? I don't see whay it couldn't be done, but I've never seen it mentioned before so maybe nobody has thought it was needed.

SRS or JPS.

It will take cards marked up as indicated, so no problem with SD, SDHC and SDXC.

I hasten to add that I'm not actually suggesting that we do make such copies of all our images!

No it doesn't. Nor do we. We never had such an obsession over our negatives and if they were lost, that was it. Digital archives are fine, but the weakness is that they need constant maintenance. Stop checking them and after a few years no one will bother with them again. Give me a box of old negatives and I might leaf through them casually and scan and process one that catches my eye. There's no such situation with digital backups.

If we want absolute archiving, then it might be best to copy all the images to archivally processed black and white film. How many old episodes of Doctor Who have only been saved because of 16mm film copies?

Age is an illusion, it's being open minded that counts, but you're unlikely to find a very young photographic society. It's just the way it seems to be. We have some younger members at ADAPS, but Atherton is a bit far from where you are.