Help on reducing truck noise
Keep in mind that low DB noise are the ones that are supposedly the less annoying. 

Try using some kind of low noise (like a fan noise) to mask the noise that are repetitive. It really works. 

But it all depends on how low your noise is. It is the honking that you probably find more annoying though. And since they come and go, they are annoying and difficult to mitigate. 

You are doing well to use different type of material and blue glue, of course.

I found this website to be a great help:

The Triple Leaf Effect &amp; Air Cavity Depth (soundproofingcompany.com)

You would want to analyze the sound frequencies (signature) of the noise FIRST. 

I recommend this:   Praat Linguistic tool for studying sounds  

You can see that if the noise gets really low (DB) you need to decouple. 

Hard to do to walls if you are renting like you are. 

As for the window and the plug, you could also decouple them by putting some kind of rubber "bead" (harden silicone) or gasket (old cut bike tube or car timing belt) that you place you plug to (which will have a frame to frame it to). It will then sit on those soft beads or rubber to absorb the vibrations. 

But, that website has a lot of good info.

2) and 3) The only way I thought of doing this would be to use some kind of board with nails or some protrusions that would in effect hold the fibreglass in place. If you use the idea on the website of the STC 63 sound reduction, you would have 2 plywood with blue gun, then insulation, then space then insulation and then 2 plywood board with blue gun. I would use (in that case) screws to prevent the FB to move. Size of plywood? The ticket the better, but space and weight might be the limiting factors.

The inside of the living space can be made to absorb (limit the echoing). I found carpeting to really help VS hardwood. Heavy curtains also helps.