opentype

Personally, I wouldn’t bother with it and just use the original image size. 
If you really need to change it, you would have to customize the email wrapper template under Customization → Appearance → Email → Email Wrapper. 
It’s this line that automatically uses the image size: 
<img src='{file="\IPS\Settings::i()->email_logo" extension="core_Theme"}' alt="{setting="board_name"}" width='{$imgDims[0]}' height='{$imgDims[1]}' /> You could use hard-coded values for width and height, for example at half the size of the image. 

No, Commerce doesn’t allow users to sell to each other. It only allows the site owner to sell products. And physical sales will removed in the future. 
User-to-user sales are not possible without third-party applications. 

Pay attention to this line in the categoryHeader template of the Listing template group. 
{template="categoryRow" group="category_index" location="database" app="cms" params="$cat"} The group name, here “category_index” points to the template set that is being used. This can be different from the category template you have chosen for the database. You might need to change that name so it matched the category templates you have customized. 
In general, your process is correct. It just looks like you are pointing to two different categoryRow versions. 

What are your Posting settings here?

Larger original attachments are actually discarded and not saved. 

Don’t get confused by the term “attachments”. Just upload the attachment and then click on the image (or the Insert button) to put the images at the cursor position. 
 
(P.S. You can’t use the Pages media images. Those are to be used in templates and custom pages.)

You need to regenerate the htaccess file in the ACP (Friendly URLs settings) and upload it to your community folder. 

Strangely enough, profile fields are not member-group specific, even though that would make perfect sense. 
You can control the appearance of profile links in the forums (but not on the profile page) by editing the “Custom Topic Formatting” option of the field. Here is an example where a link is turned into a badge. You could just replace the badge part with an image (<img src="https://yoursite.com/patreon.png">) and you would get what you want. 

You would do that on the member’s profile, not in the Gallery. 

There is no built-in option for that. It would need some custom CSS to hide the elements you don’t want when printed. 

You wouldn’t want every 404 to go to the homepage. That would confuse users and search engines. Is there something unique about the URLs of those old links which don’t work anymore? If so, on a self-hosted installation, you could target the links with htaccess redirects. 

Clear the caches. (ACP/Browser)

Certainly “working as intended”. The quote function is linked to the reply form, not any inline editing form one might be able to open as well. If what is described as “expected behaviour”, one could edit earlier posts and quote texts from posts which were made later on the page. Now that would be weird and only cause confusion. 

Wrap the ad code with:
<div data-ipssticky data-ipssticky-relativeto="#ipsLayout_sidebar"> //the ad </div>  

You could add the language string “username_desc”. It will appear automatically below the choice for the username on the registration form. 

For an image, I would probably just check the user group with PHP and then load an image with that user group as file name. 
{{if \IPS\Member::loggedIn()->member_group_id}} {{$group_id = \IPS\Member::loggedIn()->member_group_id}} <img src="https://myserver.com/assets/group_{$group_id}}.jpg"> {{endif}} Then just upload the images: group_5.jpg, group_6.jpg, group_7.jpg. 
Pay attention to caching as well. If the block is cached for all user groups, users would see the wrong images regardless of the PHP check. 
The same method could be used to load different blocks based on user groups. It doesn’t have be separate pages. 

You could always run it from the ACP in the browser. 

I am not really sure what you are asking. There is no dedicated “under construction” function. But by definition, Pages allows you to put arbitrary content on a page, and that would include “under construction messages” you might want to show. You can also use the permission system for this. So, you could work on content you hide from everyone but your admin group, while every group other than “admins” would see your “under construction” message. 

ACP → Community → Forums → Settings → Forum Settings → Topic Summary Settings

That from a Yes/No field which shows for everyone except the premium group. I have a detailed description but that is part of my own premium offers. 😉

Yeah, this can be done. Put the new field to “Member can edit”. Go to your profile and put something in that field. It can be anything. Like the word “Something”. It just needs to register content. 
Then turn off “Member can edit” again, so no one will have access that field. 
Now set the forum output to “custom” and just put your HTML in the box for “Custom topic formatting”. 
You also need to deal with the profile output. Either put your banner in there too or just put in a space or something so it overrides the regular output. Otherwise it will show the “something”. 

The number relates to the current selection, even if it is the default selection. 

It might simply not have content to show. The old method to feed this widget is tags. So when there are no matching tags, nothing shows. 

https://en.wikipedia.org/wiki/Unix_time

It’s a setting in the ACP. Put “newsletter” in the ACP search to get to it. 

There was a patch for that this week.