My own experience of using nipple shields has been that it is a load of rubbish that supply is reduced and feeds take longer. However, it took me spending half a day with BF support nurse to realise this and have confidence that I actually have a great supply, and to spot the signs that DS is full. I do think it's a confidence-in-yourself-and-your-insticts thing.
I found the link to th blog that was posted above really enlightening - all of the negatives about nipple shields is based on old studies from when nipple shields were awful, thick rubber things (my mum used them 30 years ago and said they were made of hot water bottle type rubber material!!). So hard as it is, try to stop doubting yourself and feel happy using the shields for now (and I don't see why you have to use bilaterally if your DC will feed without it on the non-inverted side! Why?! Silly HV!). I would tell myself that there's no pressure to wean off the shield until you're ready.
To answer your question about sucking on the nipple bit but not feeding, is your inverted nipple getting sucked into the nipple bit when your DC stars feeding? Or is that not happening, so nipple bit is 'empty'? That sometimes happens to me when the shield isn't placed right and 'sealed' around. Other thing is that there are 3 sizes of the medela shield (I have found medela the best BTW). Is the shield too big/ too small? Medium is best for me, even though I have big boobs and areolas - size you pick should relate to size of your nipple.
Finally, if you need some help to pull your nipple out, could you use an express machine for a minute before a feed, then put the shield on and latch your DC on?
The expressing after each feed must be exhausting (and taking all your time!!!). The sooner you can stop doing that the better you'll feel. Oh, and by the way, I suddenly stopped being able to express around 6 weeks (was expressing after a feed) and completely panicked. It was only when BF support nurse suggested it was probably because my supply was becoming more regulated (so only enough for a feed was being produced now and not more - my supply settling down) that I calmed down. It was true - if I expressed at feed time - when DS dropped one of his night feeds and I woke up in pools of milk and rock hard boobs - I got so much milk from literally 3 mins of expressing that I completely stopped worrying about supply.
Anyway, rambling a bit now, but really hope that helps. I've had so many tears about BF, horrible painful breasts, nippe shields, etc. over the last few months and now, 9 weeks on, BFing is honestly easy, stress-free and I am (almost) quite enjoying it. NEVER thought I'd say that!!!