If/when the government is shut down, the Republicans will, as a bloc, vote to keep it running (of course, because: Trump). They will need a handful of votes from Democrats to accomplish that, and last time Schumer gave them cover with a variety of (thin) reasons. This time he probably won’t.

He will. The question is whether he'll do it before the government shuts down, or after the government has shut down for a few days.

The Democrats are very likely to go for the latter. They believe that Trump's approach to governance is terrible, and his approach to financial matters is unlawful (the pocket rescissions). They want to create an emergency-level event that will garner actual press attention so they can convey that message - both to satisfy their base that they're fighting, and to (presumably) educate voters about what the Administration is doing.

And then they'll stop the shutdown. Probably with a bill that is virtually identical to the one they could pass the day before the shutdown.

I agree with you that they will get some blame for the shutdown. I think they've just reached the conclusion that it's worth it, politically. Politics involves conflict - it is the mechanism by which we make choices when those choices are contested - and the Democrats feel that they need to forcefully inform voters of what the conflict is about and that they have a position that differs from the Administration.

But from a Macro perspective, I don't think we should expect the shutdown to last for a very long time. The Democrats accomplish nearly all their achievable goals by shutting down the government for a week (or less), and after that it should be relatively easy for Schumer to deliver Senate approval.