Very welcome.

This one is interesting:

Is placing a 300x250 ad unit on top of a high-end mobile optimized page considered a policy violation?

Yes, this would be considered a policy violation. This implementation would take up too much space of a high-end mobile site's first view screen with ads and provides a poor experience to users. Always try to think of the users experience on your site - this will help ensure that users continue to visit.

There's a well-known Android news site which has ads like this so I thought it would be OK.

woody midrib: First, welcome to the forum.

There's a well-known Android news site which has ads like this so I thought it would be OK.

Second, we hear similar comments pretty often.

The thing is, with so many websites running AdSense there is probably no practical way for Google to catch all the policy violations.

I know it's irritating to come across sites violating the policies, but it happens too often to let it get to you.

The best thing for a webmaster is to read and follow the policies as closely as possible, and definitely NOT rely on what we see other websites doing.

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This will be a problem with responsive Wordpress themes. If you align an ad block on the left side of the post, it will automatically move to top on smaller screens. I have an image on that position and it appears exactly between the post title and content.

This will be a problem with responsive Wordpress themes. If you align an ad block on the left side of the post, it will automatically move to top on smaller screens. I have an image on that position and it appears exactly between the post title and content.

I'm assuming you are not referring to a sidebar ad.

Then you might want to rethink your ad position there. This is one of the few times that AdSense has come out and given a specific (and actually I don't disagree with them - when I go to a page on my phone, I sure don't want to see a big ole 300x250 or larger before I get to any content) and that makes me think this one is important to them.

I don't have ads on that position, but is very common. Even their "best practices" page recommends ads aligned to content as the best spot for AdSense ads:

[support.google.com...]

First example would move the ad to top in responsive themes.

First example would move the ad to top in responsive themes.

that all depends on how the CSS is written. If it's done well it can be positioned anywhere.
(esp. as one can ignore the crippled IE browsers as they don't do media queries anyway.

What if you have a vertical ad in the middle-top of the page and have to pass it as you scroll down through content? What I mean is, there is no content on either side of the vertical ad -- it's just the ad in the middle.

I would make sure I had some content above the ad.