opentype

It can be tricky. Many areas expect standard values or you will get cut-off letters (e.g. in the bread-crumb navigation). There is also more than one value because there is the the space between lines and the space between paragraph and dozens of CSS classes that apply in different areas.  
A start would be to override this default value and pick a higher number:
.ipsType_normal.ipsType_richText { line-height: 1.6; } This should have an instant effect on forum posts, articles and so on. 

You need to set the member group permissions on database level. You only turned on comments. You haven’t set who can comment. 

Either a custom app or a Pages database with a custom template. The latter would probably be a little cheaper as most of the functionality is already there.

It’s not an error. 
https://www.opentype.space/superdocs/ips-community-tips/pages/you-can-only-have-1-category-to-store-records-in-the-database-directly-r1/

Someone would need to write a proper import/conversion script. Those images are not just a link to a file. 

My first suggestion would be to use a NEW topic for each try. That page is now full of errors and it’s impossible to judge if a new reply would even work. 
You certainly have problems in your code. I can easily embed the forms on my forum just as Nathan.

This is NOT possible. Technically and legally. 
(It’s a bad idea anyway. If you have different projects with different URLs, menus, galleries and so on, create DIFFERENT websites, not one website where everything is somehow thrown together.)

They are just shown in a highlighted way when you type something. 

Every option would need to be its own field. 
But that only works well with two or three options, because you will get a matrix of every option combined with every other option and you would have to set up each price. 

You are probably thinking of the image proxy function. That was indeed removed. It only served a temporary purpose to not brake the page with non-SSL images by importing them into the system. 
You could copy the old HTML into the new posts and either do some find&replace each time or import it all, move the images and then do some UPDATE MySQL queries to correct the links. 
Personally, I would probably go the long way – and have done that several times while moving my content across multiple forums systems. Linked images aren’t really proper images in the IPS suite. Only “attachments” are. And therefore they are future proof. You could change your storage location and they will be moved automatically. You could switch to cloud and it will continue to work. You can change settings and the images will be recalculated … So, it might be worth it to import each image manually. 

It’s a custom.css rule to limit the size while uploading a larger image. 
.cAuthorPane .cAuthorGroupIcon { max-width: 60px; }  

There is no switch. It’s automatic. 
How that image will be used depends on the template that is chosen for the various record views. 

What are you trying to offer with the Downloads app?
It is usually used for software, videos and things like that. That’s why “screenshots” makes sense as representation of the content. 
If you add images as downloads, then it would be a different story. And the software can’t always create screenshots. For example, I have a website where the downloads consist of images, but they are huge TIF files put into ZIP archives. It would be pretty much impossible for the server to generate thumbnails from that. So, a user field to add some representative images makes perfect sense. 
 

The typical use would be to first create a custom version as Pages ACP block and assign your own key. Then you can call it as {block="your_key"}

Not, that’s not possible.
I am actually using Pages database to run my ads, but it requires a lot of customization and has to run independently from the built-in advertising system. 

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.