Ugh. No time ATM.)

It's like me :-)
There are still so many questions which should be checked, but... you know :-)

it's still a black box for me as to exactly how/when the various reward components get created out of the "new STEEM" (SBD, VESTS, and liquid STEEM)

This is one of these questions :-)
I wrote my calculation similar to the blockchain calculations. On that occasion, I noticed the conversions accordingly. However, I did not go into any further detail. Especially the VESTS conversion was not very intuitive.

where the tokens would come from if the witnesses would decide to start paying interest on SBD savings.

I have deliberately not yet dealt with this topic. But it is also a good question...

That "round" was just a kludge because Python didn't like working with small numbers.

Floating point numbers in binary representation are (almost) always only an approximation. Every language deals with them differently. Unlike C, C++ or Java, Python generally uses 64 bits for the floats. In an approximation, this can result in a few decimal places. Especially when checking for equality, you have to take this into account.
In Python, the use of the decimal module is supposed to handle it better. But I haven't really used it yet either.

Thanks for the update :-)