You may want to give the year and model of your car to make it easier to ascertain which VVT hub you have. There were different ones used over the years.

I just did the front camshaft seals on our '01 S40 so though no expert, at least the info is still fresh in my mind. My car has an early version of the VVT hub and it's located on the exhaust camshaft. I bit the bullet and purchased the locking tool. I probably could have made my own, but it was easier on my brain to use the proper tool as this was the first time I had to do this kind of job.

In my opinion, the sprockets should be removed for your head gasket repair. This is because I consider replacement of the camshaft seals as part of the head gasket job. I would not want to squish the cam cover onto the camshaft seals just to try and save $20. That's just not how they're intended to be installed. My concern is leaks after the job was finished.

The camshafts don't "plug in" to the sprockets. They're bolted together and there is no key so proper alignment is essential - but I think you know that.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig