Hi,
that's OK, no worries. As you are using 'proper' medical terminology it is difficult to figure out which terms you have been given by a medical professional versus terms you use to describe what you are observing.

- Sensation changes in the arm, such as 'pins and needles' are often not so much blood flow related as far more a sign of nerve irritation - in your case that would be the lower cervical spine.

- Quadriceps: the middle and the outer/ lateral part are always better visible - that's normal.

The way you describe your posture - maybe you should work on stretches and posture correction first before you work on muscle hypertrophy. Who is supervising your workout regime?
Are you training the right muscles??? How do you do your stretches - are you doing any, e.g. pec minor, quads?
I'm not quite sure what the goal of your training is - looks versus functional performance. Are you Bodybuilding?

If your biceps is in fact shrinking - that would be a serious neurological pathology - again very unlikely but who knows.

So, generally, I would like to suggest that you get checked out by a Physiotherapist, so that they can give you their advice (esp. cervical spine) as well as consider referring to a Specialist (in case you do have a medical pathology/ blood flow problem - which is very unlikely). They can also advise you on proper stretching ad weight resisted exercises - but for the latter, you could also ask a Personal Trainer.

Cheers,
Fyzzio