There is an old (based on Opera 12) mail program available, but it might as well be abandonware as they don't update it. I use it - have for years - but I really can't recommend it due to the lack of updates.

As that is a website you'd have to ask them, but start by making sure your language (in Settings) is set to English.

Mac is different, you have no idea what will happen in Windows.

You really should add your support to the existing request rather than adding a new one.

Weird - did you notice you also have a line on the right, but only on the bookmarks bar? Nothing like that here either ...

Where on the site?

That message means that some content on the page (such as an ad or image) is not secure, or that the security protocol used is too weak - though in this case we can exclude the latter as I would have seen a message on their landing page. Chrome would give the same warning.

Changed title to "Saving", as you can't view anything without downloading it first.

When you realized you had closed the wrong window, you should have reopened it before closing the other window. If you have multiple windows open and close one by mistake, reopening it before actually leaving Opera is easy. Once Opera has been completely closed you can lose the tabs from your earlier-closed windows. At this point no real advice to help you, sorry.

Perhaps because I use a wallpaper then? Even on Stable I have no lines.

Are you using dark theme? Which version of Opera? Here (latest Developer version) and looking at my speed dial page there is no line.