In my playthrough I find I'm able to buy almost as much as needed. Whenever I'm at a shop I'm always able to afford a fair number of bandages/focus pills, and even an adrenaline pill every now and then, coupled with weapon re-supply and a tiny bit of armor improvement, and the special deals every now and then.
This playthrough has been economy-perfect however. I've only bought exactly what was needed and no more, and always got every piece of loot on every map. In the first shop scene I didn't buy any grapples simply because they have few situations where they're more useful than an extra 5-9 health from shields (wood/steel), and you can eventually find a grappling hook in a chest later on if you want one but don't want to buy one too. (Outside the Vibra Mines mission). One thing I could see helping a bit economy-wise would simply be decreasing prices slightly, or like you said, be able to sell gear.
Like TheBouncingBunny said, the main campaign maps will auto-loot all item bags (but chests don't count), so you don't have to worry about not getting the last bag from the victory-causing kill. You can actually get two sets of Ebon Raban's armor/weapons; the first from the Coria Bridge battle (If you bridge over with the engineer and use long range attacks, or you use flying hit and run tactics, or simply bridging over and tanking with Emma/Sabrina), then the second from the city battle. Like TheBouncingBunny said too, the Stormhunter guy you get as a helper doesn't join your team afterwards, so all his gear is lost (so it's recommended not to give him anything extra). One thing to note; the halberd can only be used by a Mantis Knight, so you'd have to get one of your cavaliers to lv 20 first to use it regardless.
The boss you're talking about, Gulch, like any important bosses that show up in later missions, cannot die; they will always 'run' even though it would be nice to take them out prematurely through advanced tactics (e.g. dealing with Fera early on, while escaping from Igor's castle I think is possible), but such events would be game-breaking story wise, unless Craig takes a lot of time and effort to make multiple alternate story-lines based on which NPC bosses you kill, which I sadly don't think will happen, unless he suddenly has no bugs to fix, and no more simple suggestions to implement
As for mid-battle saves- Whether or not it would be possible to add in, only Craig could answer that. Whether or not they
should be added in, is a different story. Without mid-battle saves, it makes the game a lot more focused on strategy rather than, "oh look I didn't expect that.." *reload*.. "Ha, take that!" , where every decision you make (other than single moves), is permanent, making you think a lot harder on all your decisions.
