MODX's default "page not found" is always ID 1 unless you change that in the System Settings. Typically, you'll want to make an Error Page resource, and in System Settings change the ID to your new resource (page).
The gallery.html page... did you install Gallery from Extras? I haven't used Gallery in years, but some Extras do create resources upon installation.
There is also a system setting for a default template, you may want to check that it's set to the one you want to use as default.
As I read through this thread, I haven't seen that you've cleared the core/cache files. This would be something to do first when issues like this occur. You can do it by actually deleting all the files in the core/cache folder or my favorite shortcut to opening up the file manager is using Bob Ray's CacheClear snippet
http://bobsguides.com/cache-clear-tutorial.html
I'm going to second Bob's suggestion to not use Softaculous for installing or upgrading MODX. Really, it's only caused me pain and agony. Something's always not right. A major issue has been database collation. It's good that you've reinstalled (mostly) manually using the UpgradeModx tool but I feel that using it without following the full upgrade process could be an issue (I'm a 'lil paranoid. I also like to do upgrades old school style and just extract MODX.zip on the server. It's not exactly difficult or time consuming). So even if I do use it to upload the new MODX files, I also make sure to do all the things such as updating extras and clearing the cache as is found here:
https://rtfm.modx.com/revolution/2.x/administering-your-site/upgrading-modx
Another thing to consider is your rewriteBase in .htaccess. It appears that this domain is a subdomain or is living in a subfolder of a WP site, no? If so, make sure to change:
RewriteEngine On
RewriteBase /
to
RewriteEngine On
rewriteBase /subdomain/
(not sure how a subdomain (or site located in a subfolder of public_html) would even work without that but it's worth a mention)
I'd also suggest checking your configuration files for the correct paths. If you look about half way down on this page, there's a list of them. These also should show your subdomain/subfolder in the path. Make sure that the full path to the MODX installation in the subfolder is set correctly in all the files.
https://rtfm.modx.com/revolution/2.x/administering-your-site/moving-your-site-to-a-new-server