"Nodes" trying to enforce rules without majority hashpower on their side had better change the PoW

If there's unanimous consensus among full nodes that a softfork (or hardfork for that matter) should happen, miners have incentive to enforce those rules, otherwise they will mine invalid blocks that they won't get paid for.

or else they can (and arguably should, for sanitation purposes) have their ledger wrecked by doublespends by that majority hashpower

Sure, if they are this malicious, ithere's no point in keeping them.

Software wallets (what you are calling nodes - of what network? certainly not the Bitcoin mining network)

"Bitcoin mining network"... sigh

Instead the economically signficant people and businesses, who may happen to run "nodes," exercise a soft power: the power to incentivize miners to agree with them.

Bingo.

Miners excercise the hard power, the ability to kill chains, but they never want to use it unless they feel the chain is destined to be in the minority.

Exactly.

It is our (users of SPV wallets) risk if 51% of the hashpower SPV mines, as it becomes the same as the attack condition in the SPV section of the whitepaper.

This is very true... SPVs are not perfect today.

See, Core never understood why SPV security is adequate

Core is not a person.

so they fail to see how Segwit breaks it.

I fail to see how too, explain what you're talking about.

Either that or, putting on lobbyist hat, someone wants to ensure SPV security is not adequate so that everyone must run full validating "nodes" (wallets)

Aah, here comes the conspiracy theories. Everyone running full nodes is far from practical today, but sure, that's a good mission.

so that blocksize cannot be raised much, so that the stream of transactions is kept to a trickle so as to provide incentive for their off-chain solutions.

More insane conspiracy theories.