Confused about input and output spending limitations and consolidation
I spent some time researching the problem and learned that all money in an address must be spent at once.

This is incorrect. The concept of an address does not even exist at the protocol level.
A transaction has inputs and outputs. Every output constructs a &quot;coin&quot; which has an amount and a locking script (encoding the conditions for spending it, which is often - but not necessarily - representable as an address). Inputs consume coins (in their entirety - every coin can be spent exactly once). Transactions can be seen as melting down old coins, and reforging new ones with them.
There are no rules about the locking scripts of coins you're spending. They can all be the same, they can all be different, and there may be other coins with the same locking scripts that aren't spent by the same transaction. Simply put, the Bitcoin protocol has no concept of &quot;all coins associated with the same locking script&quot; - this is only added by wallet software and block explorers for human consumption.

Is this configuration valid? Or will there be an error when I try to commit the transaction? Or will all the other inputs to the address be permanently lost? I am too scared to try it.

It is valid.

And why was Electrum moronically trying to force me to spend all of the address at once, phenomenally increasing the fee when I was perfectly happy leaving the rest of the damn money exactly where it was. And why are there so many articles online explaining why you have to spend inputs all at once?

I can only speculate here, but one reason may be privacy. By definition, multiple coins associated with the same address/locking script can be spent by the same people. Thus if you spend in multiple distinct transactions, you're revealing to the network that those two transactions are related to the same entity (you). Because of that, it is preferable to always spend all these coins simultaneously, the minimize how much you reveal.
Of course, in an ideal world you don't reuse addresses, and you never have multiple coins associated with the same address in the first place (except perhaps dust spam).