7. There are 3 weapon sets you can switch between, in the inventory or using the hotkeys F1, F2 and F3 (by default).

8. You need to provide the alchemy formula to both alchemists, other than the ingredient free, level 5 stat potions Barbatos can make, or Allan's extra strong resistance potions.

9. Both trainers teach the same sills, to the same levels; their 'specialties' just determine the price of upgrades.

Any fights that are difficult generally can be dealt with by upgrading equipment or skills, adjusting approach, or doing another area first and coming back at an appropriate level.
Amdusias is level 15; fighting him at lower than level 15 (so without access to the level 15 tier skills) is more challenging than at 15, and being higher than 15 makes the fight easy.
In DKS you need to hit both levers to disable the regeneration chambers to defeat General Raze. Being stunned will not prevent him from reaching the chambers (there were various techniques in the original release that could make the fight trivial, like just blocking access to the chambers, so to fix that, he basically cheats [teleports directly to the chamber in the middle of a Thousand Strikes move, etc]).

You shouldn't get killed in one hit even on nightnare; that sounds like a bug: Ba'al. The Impossible foe?).

After taking over the battle tower you have a chest there to send things to (just sending alchemy and enchantment ingredients there should give you plenty of room in the inventory).

I usually kept quest related books (even if they were just added for info or atmosphere), but sold ones that show up in randomly generated loot, etc (like the different 'History of' book series). If you check the consumable section of your inventory, you may have a stat/skill book there you can use to fee up some space. Any required books should be in the quest item section of your inventory, though there are a couple books which you can read yourself to get a bonus or give to someone to complete a quest. In any case, after you've read a book you can sell or destroy it (as long as it is not marked as a quest item, in which case it doesn't count towards your inventory limit).

If you have any creature parts you only need to keep the best of each type. I kept a list of my current top part stats handy so I could sell any lesser versions before talking to the necromancer (and working on the creature), so I wouldn't clutter up the part list when any new pieces I had with me were added. In DKS/D2:DC creature parts can also get added to the necromancer's list from any merchant's inventory, if you sell or buy anything in that section of the inventory (and maybe just switching to it).
You can get rid of potion and enchantment formula by talking to the alchemist and enchanter (any left in your inventory after going into the alchemy or enchantment functions will be duplicates, and can be sold).

Alchemy plants and enchantment gems and minerals are easy to acquire after getting the battle tower, so you can get rid of any you are not going to use. Keep malachite gems, since there is a limited supply in the game, and ginseng (etc) for healing potions, but if you have multiple stacks of the same item, you can safely get rid of the extra stacks. Iron, whisperwood and fatfern are fairly common, for example; you can make more money with the latter two if you make level 1 or 2 Full Resistance or Dexterity potions, and sell the potions rather than just the ingredients.

If you have any equipment that you are hanging onto because it might be good once you get higher level enchantments or better charms, by the time you do you will most likely also have found equipment with better base stats (my first game in the original release of D2:ED I hung onto lots of stuff I ended up just selling later).

There is a skill to boost inventory space, if you have the points to spare. You can unlearn all your skills after getting the battle tower, and get those points back once you have access to the chest there to store things. I maxed this still in my first game, and didn't learn it at all in my second.