I don't have flat or inverted nipples, but at the start of BFing DS one of my nipples got torn off so there was only a big gash where it was. The LC's I saw (after 7 weeks of healing but still no nipple present!!) suggested using a pump to draw out the area around the nipple so it was in a shape that DS could find. So maybe - since you are using a pump at the mo to encourage let down with the shields - you could try just latching to the 'cone' shaped end of the breast after the pump, if you wanted.
I believe there are also little contraptions that are designed to bring out the nipple. Again I have not used one so I don't know if they work or not. I believe Avent have one out but you can find them on Amazon at any rate and then see if its a brand we have here.
congratulations on persisting so far. I know how hard it is to keep going when things keep going not as planned. Well done