The resolution of a lens at a given wavelength is determined by its diameter (Rayleigh function). We know how big the launch vehicles are, so we can estimate the largest size a spy satellite's mirror could be, and we can use that to compute the maximum resolution a satellite could have; it turns out to be something around 5-10 cm. In order to resolve a newspaper from near-earth orbit, you'd need a lens bigger than the ISS, and if such an object existed it would be one of the brightest objects in the sky.

Would it be possible to improve the resolution of optical imagery using a synthetic aperture?

Yes, aperture synthesis is possible with optical wavelengths. I don't know how practical it would be to actually use with spy satellites, or how much of an improvement they could see with it.

Keep in mind that there are a lot of people that track satellites, even spy satellites, as a hobby. I haven't heard of anyone discovering two or more satellites orbiting in the sort of tight formation you would expect would be required for this.

Beyond my knowledge, sorry.

Yes, much finer optical resolution is possible if you use multiple instruments separated by a controlled interval.

In radioastronomy, there are arrays of dishes that use this technique. There are good pictures in the Wikipedia article. [0]

One military implementation was a connected three-satellite constellation, built by the US Navy for scanning the surface of the world's oceans. [1]

But those tools work in (relatively) long wavelengths.

I have never heard of free-flight multi-aperature interferometry that works in optical or near-optical wavelengths.

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[0] "Very-long-baseline interferometry" https://en.wikipedia.org/wiki/Very-long-baseline_interferome...

[0] "How VLBI Works" https://en.wikipedia.org/wiki/Very-long-baseline_interferome...

[1] "White Cloud | PARCAE | NOSS" https://en.wikipedia.org/wiki/Naval_Ocean_Surveillance_Syste...