If you have Volo in your camp, you can trade with him.
Also, for your spell slots can you sleep first to restore them before meeting Halsin, or does the quest fail?
Yes I have Volo in camp though not by choice. I never spoke to him in the Grove or the goblin camp but hey, Larian choice. Volo has hardly any gold if I'm not mistaken. I know I can sleep to restore spells.
I'm sorry, but the game clearly states, that you should finish all unfinished business, before moving along.
No it does not - not even close. What it actually say is:
"Are you sure you want to proceed? Depending on your choices, the state of the region could change and some active quests may become unavailable". There is absolutely nothing concrete in that warning and certainly nothing about any particular quest or specific place or NPC(s).
What exactly does 'the state of the region' mean? Which region given that you are about to step into Shadowfell? There are also a lot of choices in the Nightsong sequence - Which party member does the talking; what you do about Balthazar; what you do with Nightsong and what happens with Shadowheart afterwards if she is present. That's a lot of permutations involving a lot of information that you are unaware of when you get the warning. You cannot make choices about things you don't know - that is guesswork not choice.
In any case this has nothing to do with my gripe. My gripe is that losing the portal battle ends the 'find a cure for the curse' quest which is nothing to do with what is going on with this warning, or at least as far as I can see. Maybe I am missing something here?
It is the failure of the find a cure quest that annoys me not the death of Halsin or even the failure of the Art Cullagh quest. Am I to believe that completing the game by defeating all these major bosses doesn't not remove the curse because Halsin died? Who exactly is/was responsible for this curse?
My comments about the the Act 1 waypoints being removed was related to the lack of logic between what happens with the Nightsong in Shadowfell and Act 1 waypoints in the Hinterlands.
The concept of some areas, quests etc. being cut off at a certain stage of the game or after certain acts is not an innovation by Larian - it has been around for a very long time.
And yes, the game has consequences, that is, what many people here like. I forexample took too long to look for an npc and said npc died on me. I was sad, but I took it. There will be more situationes like this going forward, just as a warning.
Again this is not a concept introduced by Larian. Most of the games I have played make a much better job of it than Larian and don't force consequences on you regardless of the choice you made.
