[levin]

1)  In the following rare case, you may not need a whole screwdriver.  If you use the L-wrench included with the preassembled Tangara on the top screw, then you can use another Torx T6H bit to hold the back screw still with respect to the front screw.  Alternatively, if you want to live life on the edge regarding risks, then you can hold the back screw still with a 1.5mm flathead bit - but be extremely careful so that you don't strip the screws on your Tangara.
However, at this rate, I'd strongly recommend buying a screwdriver kit with the most common bits.
I have the iFixit Mako Driver Kit, but any screwdriver bit set will get the task done of fastening the outer 4 screws of your Tangara, as long as it has the Torx T6H bit.

That makes sense. I ended up ordering iFifixit's Essential Electronics Toolkit, that should also have the right bit. Will report back once I have it...

2)  I haven't experienced this issue - perhaps the plastic around the top-right screw could be somewhat structurally compromised?

I hope not! I suppose I'll see once I'm able to open it up.