I don't think it's possible to access all of the dialog options with a single party, since there are some extremely specific ones (off the top of my head I know there's a side-quest that requires a Transmutation specialist Wizard with a high alchemy score). However, you should be able to cover most of the major stuff with the party you have presented there. Covering your combat needs is definitely top priority, and having a Half-Orc Barbarian backed by a Cleric should give your party the backbone to handle the game's more difficult challenges.
Human Paladin X/Fighter 4 (Longswords)
Looks fine. Just be mindful that Paladins are stretched quite thin in terms of attributes. You need strength for melee, charisma for paladin class features, wisdom for paladin spellcasting, constitution to not die, and your dexterity needs to be at least passable so you don't take an AC penalty. That's 5/6 of your attributes... these guys stretched very
thin. You'll probably get more value out of staying single-class Paladin, but there's nothing wrong with taking a few Fighter levels for extra feats.
Half Orc Barbarian X/Fighter 4 (Some twohanded weapon, good suggestions?)
Probably the strongest melee build possible, which can be very helpful in getting through some of the game's more unforgiving boss battles.
There's not much point in specializing in a specific weapon, and it's usually more advantageous to have 3 or 4 different weapon options available to pick the best one for the battle you're about to enter. Fully-invested weapon specialization is only +1 to hit and +2 damage, and you'll often be able to get better benefits than that by swapping to a more advantageous weapon type. Weapon specialization is more important if you're running a low-damage build where every damage boost counts. Barbarians don't have that problem.
If you do find a weapon that you absolutely love, by all means specialize in it, but there's no need to go into the adventure with the intent to specialize.
Elf Morning Lord of Lathander (Longbow or Mace/Shield, Support & DPS caster)
Solid; you can never go wrong with a Cleric. You may be stretched a little thin to do both melee and ranged combat, but since that's a secondary role for this character it's not going to be the end of the world if you're a bit below the curve. Just be mindful that longbow attacks don't scale well in IWD2 and can't output damage effectively at higher levels. Still a decent weapon if you want to keep the Cleric in the back.
Elf Bard (Longbow and I love Bards)
Another solid support character. Same pitfall as the Cleric, be mindful that bow isn't going to be doing much damage at higher levels.
Dwarf Fighter X/Thief X (Dualwielding daggers or shortswords, obligatory trapdisarmer)
Rogues sadly suck in Icewind Dale 2. Fighter/Rogue is great in most of the 3rd edition games, but IWD2 isn't kind to it. Its combat abilities get nerfed, its non-combat abilities don't get used frequently enough, and the traps in this game are almost without exception easy to disarm so non-rogues can handle the job with the cross-class skill system. I ran a Fighter/Rogue in my first playthrough of IWD2 and was so
disappointed with her that I actually ended up restarting. I used a Wizard with cross-class skills to disarm traps and had the barbarian manhandle any locks. I think there was one
trap in the entire game that caused me serious trouble with that approach.
Dual-wielding is a bit mediocre in IWD2, but it is doable. Ensure you maximize your strength, since IWD2 is stingy with other forms of bonus damage. Going the dexterity-route may look intuitively appealing, but in practice there's no way to boost your damage so it's like chipping away at an enemy with a nail file. Go the strength route so you can hit hard.
Halfling Sorcerer (Crossbow, offensive caster)
Sorcerers are great, although I would personally recommend you pick up some battlefield control spells to complement your damage spells. Spells like Web or Stinking Cloud can disrupt the approach of enemies and let you pick them off at your leisure. This is a more useful opening spell than a fireball, since enemies that have taken some damage are still threats to the rest of the party but enemies entangled by magical webs or knocked unconscious by noxious magical gas aren't. This can also be useful for getting enemies to hold still for a clean shot with the fireball ;-)
ask questions like these? No. Play the game *your* way. Don't ask other people how to.
I find it deeply ironic that you're telling him how to play/enjoy his own game here.
Not everyone enjoys going in blind, and a lot of people do like to refine their party in accordance with their preferences. Asking for advice to better tailor your own party to the kind of game you want to enjoy can be "your way" of playing if you so choose.
your priest might need to spend a feat to be proficient with her bow; not sure about the bard.
Elves get proficiency with bows for free.