One point about the timing: Arming Ukraine had become a football to kick around in US politics. Under Obama it was still considered too provocative, but now we were at a point where not doing it was basically borderline treason. So if you want to drive to Kiev today is better than tomorrow.

As to what the Russians want to achieve, I guess the best case is they think they can do a repeat of Georgia, have Zelensky implement something very much like Minsk 2 with additional territorial losses and go home. Worst case they try to go for regime change, which would have to be backed by a substantial military occupation.
The thing is, can scenario one really work out long term for Russia? Georgia is far off, while Ukraine is bordered by NATO countries and being victimized by Russian aggression has basically been turned into the founding myth of, at least the western parts, of post Soviet Ukraine. Which is why I really didn't think the Russians would go for it. (with a side helping of US intelligence haven't been right about anything substantial in the last thirty years). But then, if the policy is really driven by Putin being an irredentist right wing crank we're definitely looking at much worse outcomes.

As to Western intelligence: I find it notable that Zelensky was complaining that no one told him anything and apparently the German intelligence chief had to leave Ukraine with the first wave of refugees while the anglo spook connected journos were confidently talking about an exact date for the invasion. Was there a Russian ultimatum to the US? Putin reportedly considers talking with the Europeans as a waste of time, an estimation that is famously shared by the US ("fuck the EU") so the Euros being out of the loop wouldn't be surprising.