It's doubtful Snowden was in possession of his NSA data dump at the time he arrived to Moscow, the things he had memorized would have been of very limited value.

If the Russian government was in possession of his data, I'd consider it fairly surprising that they seemingly never leaked any of the materials.

While it's not strictly impossible that Snowden through the Russian Government was the "second source", given that all the leaks from the second source came after Snowden had landed in Moscow, none of the "second source" files were included within the Snowden dump a bunch of journalists have access to. There are also various more specific reasons to belive that Snowden probably would not have had access to all the things originating from the second source, and even more so many of the things originating from TSB.

Same is true of Snowden possibly being TSB, whether or not "second source" and the TSB were the one and the same. It's just not really credible.

Here's a good starting point if you're not familiar with the second source https://www.electrospaces.net/2017/09/are-shadow-brokers-ide...