I figured some things out.
Two different block themes give white screen of death on subsite if I use the link; Appearance > Editor (beta) in admin menu which is this;
/wp-admin/site-editor.php
The above link in wp-admin always adds &postid=
the postid of which varies depending on theme.
There are no links in wp-admin to these site-editor sub pages but I can get to them just fine;
/wp-admin/site-editor.php?postType=wp_template
/wp-admin/site-editor.php?postType=wp_template_part
I can also get to any Template or Template Part from there. That means anything can be done if bypassing the appearance>site-editor link.
Running the same block theme on both main and subsite, made changes to the Header and Front Page Template on both and as mentioned in my previous post, those changes are done in the DB so one has no affect on the other and either one can be undone.
Swapped both to a different block theme and got the same results. Tried yet a third block theme and did not get the white screen of death on;
/wp-admin/site-editor.php
So, bottom line is, yes you can use a block theme and make changes to one site of a multisite and it doesn’t affect the others BUT, you may get the WSofD.
I keep these two links handy…
/wp-admin/site-editor.php?postType=wp_template
/wp-admin/site-editor.php?postType=wp_template_part
…plus links to customize, nav-menus, blocks(reusable) and widgets because most block themes hide those even though they still exist and customize especially is useful for the Additional CSS section. Nav-menus can also be handy because it’s quicker to build a menu there and then select it from the block editor, Navigation block.
Likewise, I have a KB plugin that has it’s own sidebar with widgets, regardless of the theme so I need to be able to get to wp-admin/widgets.
Any of the above are a lot faster than that main FSE page and none give you WSofD.
Copy this, add your example.com in front and keep them handy for block themes.
/wp-admin/site-editor.php?postType=wp_template_part
/wp-admin/site-editor.php?postType=wp_template
/wp-admin/edit.php?post_type=wp_block
/wp-admin/customize.php
/wp-admin/nav-menus.php
/wp-admin/widgets.php
Of course, keep in mind, beta so ymmv and should probably not be used for critical sites.