Frank, to your questions:
1. Yes
2. Try viewing the theme in IE11
3a. Yes, according to the developer
3b. I don't know, I hope not

I have discussed this with key people at CC, and we seem to have different views. I would have wanted to have a full fallback, and then go for grid, but I'm not so good with grid (yet), so I'm struggling a bit.
Apparently the %age of IE11 users is now between 2 and 3, but my own experience with a site I have made (pre-SD), it happens to attract a fair amount of Chinese visitors, and they use IE11. Globally the percentage might be low, but there are 'pockets' with much higher usage of IE11. I haven't actually checked with caniuse.com if there are other browsers which cannot handle grid.

Thanks for that, Inger.

Regarding item 2, it is indeed a mess in IE11, with the 'Lorem ipsum' panel incomplete, the 'Iconc (sic) Designs' panel partly hard to read and overlaid with items from other panels.

The usage share of browsers depends on the devices being used. This page for November 2019 (in the 'Summary tables' item headed 'Usage share of desktop browsers') showed IE as exceeding Edge and Safari, and only two or three percent behind Firefox.

I therefore intend to stick with what I thought had been considered best practice in SD: doing a Fallback, and using CSS Grid where there is good reason to do so. I'm not sure there was anything in the Trends theme that required CSS Grid.

Frank

The only way to correctly handle fallback when using CSS Grid is to replace the html structure if the browser does not support it. Period.

I think we all agree on that, but in order to have a fallback at all, the page elements need to be placed on the canvas before turning any containers into display grid. If you compare e.g. the two themes 'Gloss' and 'Trends' in IE11, you will see that in Gloss it has been done correctly, in that display grid doesn't kick in until you pass the first breakpoint, whereas in Trends it looks like display grid has been selected first, and then the elements have been added. That is why Gloss is still readable and workable in IE11, while with Trends that's hardly the case.