Will nuts and hexes be enough to protect the traverse on heart of darkness? Or will I need some cams?
Depends how many grades you have in hand. I have no doubt you could do it all on passive gear (it's a joggly horizontal, there are big nuts and threads the whole way) but you would potentially be absolutely boxed by the end of it. First pitch is all on your arms and I'd really rather be banging medium-big cams the whole way.
You also run the risk of pieces wangling out when you pull the ropes, which would make it a very serious proposition for your second indeed.
Second pitch is piss.
The HS finish is bird banned at the moment as well, so you'll need to be steady e1 to get out.
After my friend failed to get very far on sealhut and had used up most of our large cams (we had no hexes). I decided the best option was to do heart of darkness instead finishing up Blowing in the wind finish due to birds. This was because as I had done Sealhunt recently but hadn't lead the first pitch of HoD previously. Needless to say the 5m plus run outs between the gear made it rather exciting for both parties as I tried to spread only 2 pieces of large gear i had along the whole pitch. We probably got pumped as we were likely over gripping and alternative gear was fiddly to arrange and remove. It was boiling so we were fried by the time we eventually topped out, but it was a quality adventure and a good challenge. I'd recommend cams too and plenty as previously suggested though I guess as previously mentioned it is possible without any, and was perhaps the norm when first done. The break also can get a bit smeggy so it is not best done early in the morning if there has been any dampness overnight.