I cannot find a direct proof but since ToEE uses 3.5 edition rules we can do the math ourselves. You see, effective party level is NOT just the average level of your party members! You calculate it as follow: sum of levels of every character in you party divided by the STANDARD party size (and not number of party members) In 3.5 rules standard party size is 4. A few examples to make it clearer:
- If a party consist of 4 members, each level 5 then effective party level = (4*5) / 4 = 5 (fairly easy)
- If a party consist of 3 members, each level 5 then effective party level = (3*5) / 4 = 3.75 (because this party is significantly weaker than the standard 4-member party)
- If a party consist of 5 members, each level 5 then effective party level = (5*5) / 4 = 6.25 (this party is obviously stronger)

In your case I'm fairly certain that your effective party level is not 2 as you think but: (5*2) / 4 = 2.5
If you let NPC (level 5) to join your party then you will have (5*2 + 5) / 4 = 3.75
So yes, it will definitely influence your XP reward.

The only problem is that (as I mentioned before) I cannot post a direct proof of it. However, ToEE is known to use 3.5 rules quite faithfully and the manual does mention that XP reward is scaled with party level so I'm certain it follows those rules I described above. I can imagine that perhaps "standard party size" is different in this game (for example 6) but it doesn't change a fact that getting a new NPC will change your XP reward. But I still think it's 4.

As for the second question: I don't think so. I finished the game and no NPC was in my party, even for a few seconds. I don't like NPCs ;)