GW are not noted for their generous licensing terms; very much the reverse, in fact, judging by conversations I've had with owners of small businesses interested in trying to sell their physical products over the years, and they insist on a huge amount of control over exactly what can be sold, how much stock the business HAS to take per month (regardless of what the business itself knows will actually sell), and how it can be displayed. I can't imagine they'd be any more willing to consider something like this. If they were interested, I'd expect they'd be doing it in-house already.

It shouldn't create a misalignment if you use the snap grid properly, but you may need to mask the second grid in places. Or you could just draw one grid across all, and mask those areas of the grid that lie beyond the structure's walls.

As with most things in CC3+, there isn't just one option for solving issues like this - it's which one you're more comfortable with that typically wins the day.

If you're finding the mapping a drag, and not really enjoying it, it may be worth considering a broader-brush approach to the city overall - such as the Watabou style, that shows general areas only, with just a few important places highlighted, roads and general layout. Then if you feel so inclined, you could add detailed areas for selected spots as separate maps - that way, you don't need to worry about how the map looks zoomed out, and can pick area sizes to better suit the time and energy you have available for detailed mapping.

As Sue said, what you've shown here looks good in detail, but if it's not inspiring you, it may be worth a rethink, if only for next time.

Well, I'm at the stage now where I'll probably just get to be usefully familiar with about 25% of CC3+, and then they'll move us all to CC4 😎!

Looks nice!

Maybe a bit more contrast for the river and road lines in places, so they pop better on the greyscale?