Can you get another acceptable offer to use as leverage? If you can, go get one and hang onto it. Have a conversation with your manager, sans letter, and mentio

Can you get another acceptable offer to use as leverage? If you can, go get one and hang onto it. Have a conversation with your manager, sans letter, and mention that you aren't happy with how things have been going, how you are wearing many different hats but don't feel fairly compensated given how heavily the company seems to rely on you. See what he/she says. If things go well, great. If not, wait about a week and then bring the new offer to your manager. Mention that you're not thrilled with how the company doesn't seem to share your concerns, which is a shame given your history with them and how interesting you find the work. Tell them that you don't want to have to leave, but you need to know that management takes your concerns seriously. If you can't get actual change that would make you happy pushed through in about a week, take the other offer and don't look back - they're just stalling and politely ignoring you. Otherwise, you have to decide whether what current company does is good enough to pass on your new opportunity. If you play your cards right, I think you can get by without seeming like a 'traitor.' You seem like a smart enough guy to sort this out yourself, so I doubt I'm saying anything new. Regardless, I hope it helps give you a little bit of clarity.