This article from August of 2019 states that GarageBand supports audio unit plugins.
So you still should be able to "import" third party instruments. Maybe it just needs to be pointed at your directory of plugins.
Here's another link from Apple regarding re-downloading available sounds ( maybe the upgrade nuked the old ones? ).
I think the thing you are looking for is ReWire, the Propellerheads protocol to send audio from app to another application ( ReWire Client -> ReWire Host ). Low Latency audio in windows means exclusive audio ( 1 application at a time sending stuff to the sound card ). So you would have to see if your "Midi Studio without Support for VST mode" ( Reason?) ...
You can use the USB interface for the output ( you'll need ASIO4ALL most likely to get low-latency ) but for playing your Guitar into the PC you'll need a different interface that can accept high impedence inputs as mentioned by "Your Uncle Bob" in the comments. A low cost option for you would be this:
TLBX has a free online Music Scale Finder which prints diagrams for different scales on a variety of instruments, including guitar. And you can use the “share” button to get the code to put the diagram on your website if you blog about music theory or something.
the exported track does not sound much peaky and sounds OK, but why do the waveforms say otherwise?
Each software can render waveforms differently, so I don't think judging audio level by inspection of the waveform is an accurate idea.
For example, in Reaper you can alter the size of the waveform. You can make it look as big or small as you want. Of course,...
Without having listened to your track: it's not surprising at all it looks this way if this is just a bass-and-drums loop with not changes. How could it look anything but constant-peaks-amplitude, if it is a loop that literally repeats in exactly the same way?
The fact that this constant amplitude is actually 0 dB (i.e. the maximum possible) is just a ...
Sibelius doesn't play it back one octave higher when you click on the note.
You have to start the playback ("P") from the bar where the 8va sign begins.
If you start the playback in the middle of the 8va, it won't be rendered.
I came here from Google, didn't find any solution if not by myself.