Belle you're right that at the moment the limit is £23,000, however that excludes your house. It's is not considered an asset for care liability. So it's £23,000 in cash. The £100,000 includes your house, which means many, many more people will be liable, as lots of homeowners have a house worth more than that, but not necessarily large cash savings. Currently they are still leaving the value of their house to their heirs, this will now be eaten up.
The scheme extends to all those who need care who don't currently pay, if they have over £100,000 in assets, including house and savings. But it's likely that those with long term, slow conditions, like dementia, will end up with much higher bills.
Happy I'm not sure, I believe local authorities have freedom when managing these schemes, so it's possible that they could run a scheme where interest continues to be run up on the debt. They could argue that you have the option to sell immediately and pay up front, rather than delaying and accruing interest... but yes, if there is interest added the debts could be massive.
It is perceived to be a shared expense, like most household expenses. If your partner won't pay for your care who will? (Rhetorical. Surely that's what we pay NI for. But apparently not.)