It sounds fine, just monitor the way it turns out
Nah, just run it as written, assign xp as written and adjust as you need to
Here's some suggestions for the "adjust as you need to" step:
-If there's four or more enemies, feel free to remove one. Maybe even two if you feel there will be a problem, or if the enemy is 8+.
-For single tough enemies, you could apply negative levels to weaken the enemies a bit. Rather than rebuild them from scratch, this is a quick and easy way to rebalance encounters.
-Alternatively, you can not touch the stats, and just have the enemy "go easy". Maybe it doesn't use it's full attack routine even when it could. Maybe the bandits don't use the Potions of Invisibility they are carrying. Just don't fudge the dice, that's dangerous.
-You could provide the players with an NPC ally that is not particularly optimized but competently fills in a missing role. I like Healers, Bards, and Sword+Board Fighters for this role. Alternatively, you could take the opportunity to let them befriend some sort of monster as a fourth party member. Finally, if due to the lack of players the party is missing healing, it's not an awful thing to provide players with say, a totem that has a few CLW spells per day. That staves off the 15 minute day syndrome.
-If your players are experienced players, however, you shouldn't change much of anything. Experienced players will generally be able to make up for any weaknesses in surprising ways. For example, if I knew we were down a man and was having trouble with encounters, I wouldn't need to be provided an NPC, I would go search one out. Experienced casters can solo encounters at level 1 with the right spells.
As far as XP goes, I agree, don't change the XP (unless you remove enemies entirely). Players will level up faster than normal, which will actually get them more or less on track eventually.