This conflict may be a disaster for Ukraine, but how is this conflict a disaster for Europe?

Is Europe going to ratchet military spending at Putins's bluff?

If you,Europeans believe that Russia is such an existential threat,why not attack preemptively ?

The money is coming from increased debt.

Maybe... but probably not. Having to divert investments from one part of the economy to another is not that much a big problem: Russia has been doing the same and they have an economy of war that works more or less (some say they are on the brink of collapse and yet, they are still there). So, Europe can totally rely way less on the US, they just have to change their priorities, and they'll adapt just as Russia has adapted. Thinking they cannot is really presumptuous, or even comptemptuous (and a lot of people have made the same mistake with Russia by the way). And yet, at the moment, the US think that way, not believing in soft power any more, but only in pure pressure or even blackmail. If history teaches one thing, it is that you always create your own ennemies (Versailles treaty comes to mind).

"Europe has benefited a lot from not having to pour tons of money into defense spending."

I'd say spending so much on American weapons has hurt it's own domestic capabilities if anything.

Bold of you to assume there will be a next administration in the United States in four years.

"hope"