The number of ads you can show has been limited.

 

1) Will Google correlate the increase in clicks with seasonal traffic + Google Ads campaign traffic?

Seasonal spikes are OK, provided the traffic sources are considered valid.

a very successful Google Ads Display Network campaign.

This could be your problem. Traffic from advertising is often invalid. AdSense works best with organic sources.


2) How long do these policy suspensions usually last? The online documentations are deliberately vaugue.

They last as long as it takes for AdSense to be satisfied your traffic profile is safer. That's why the documentation is vague ... it depends.


3) Is there some mechanism to appeal this or get a human involved sooner that I've missed?

No. Invalid traffic algorithms are fiercely guarded so you would only get general advice even if you could appeal. And what would the basis of your appeal be anyway?


Common reasons for invalid traffic are, in my observation:

ads
social media links (particularly self-placed ones)
any other non-organic traffic generation ideas
ad placements that lead to accidental clicks (eg clickable elements on your site being too close to ads) (see: https://support.google.com/adsense/answer/160534)

 Make sure you don't have more than 3-4 ads per page and that you keep 50 pixels space between everything that is clickable to the ads. Once you have enough traffic for google to estimate your changes, everything will go back to normal.

Remove ads.txt file from root directory?