The only effect will be not completing Wesson's second quest, if you picked him as your enchanter, and not getting the achievements for finishing off all the flying fortresses and possibly fully upgrading the battle tower.
If you go back when you've done everything else in the fjords and are ready to enter the Hall of Echoes, being at a higher level with better equipment (ie higher DPS) should help with General Raze.
In D2:ED there were a couple exploits that could trivialize that fight (letting the weak Walking Armours build up, and standing in or in front of a regeneration chamber so he couldn't reach it), so the 'fix' in DKS/D2:DC was to make the armours stronger and have the general 'cheat' (run away when stunned by a Stun Arrow or in the middle of a Thousand Strikes attack, etc, or teleport directly to a regeneration chamber, even if it was occupied). The key is to get to both levers quickly to temporarily disable the regeneration chambers, and then do enough damage quickly to kill him before they come back online.
You can have the creature and one summon active at the same time, and can charm one opponent. You can not have more than one creature, or more than one summon active at a time.
For a charmed opponent, the experience you get when they are killed depends on the amount of damage you (or a summon or creature) do, compared to the amount the enemy did when they were charmed. For example, if you charm an opponent, and the other opponents take off 40% of its hit points before the effect wears off, when you kill it you would get 60% of the experience you would have otherwise gotten. While this can mean slightly slower levelling, it can be quite effective, and more than make up for this 'loss'. Summons tend not to give experience anyway, so if you Charm an opponent's summon, there is no difference with experience.