I think using the term "Action" is causing problems. It seems like you're using it as a synonym for "do a thing" and in 5e, it's a specific term with defined meaning.
It's important to understand that Move is very different in 5e than 4e - it's not an action at all. You can split up your move in any way you want, using it before, after or between your actions. So you can move, attack, move again, use a bonus action, then move again, as long as you have move left.
5e has "Action," "Reaction" and "Bonus Action" and you only get one of each of those per turn. There are some exceptions on Actions (Fighters can get a second Action with Action Surge) but there are no exceptions that I know of for Reaction and Bonus Action - you get only one of each of those per turn.
What this means is that you need to make choices.
Action: Move 10 feet to perform Sneak Attack with Shortsword getting 2d6+3 (extra 1d6 Sneak dmg).
This is two "things" - using part of your Move, and your Action - an Attack. As pointed out by Greenstone Walker, you don't get the Sneak Attack with a Longsword since it's not a finesse or ranged weapon.
Action: Offhand Attack with Longsword + 1 getting 1d8+1 dmg. (<-- does Sneak dmg apply here too?)
You can't do an offhand attack with a Longsword, as noted in other answers. But you can do an off-hand attack as a Bonus Action if you had something small (like a second Shortsword). You don't get the Sneak Attack damage on the second attack, nor do you get your attribute bonus to damage.
Action: Disengage (Cunning Action lvl 2 Rogue)
Cunning Action lets you use a Bonus Action to disengage, but you can't - you used it up with the off-hand attack.
Action: Moves 10 feet back to Pillar.
This is not an action, it's just more of your Move. You're going to take an Opportunity Attack, since you didn't Disengage.
Action: Use Item: Drink Potion of Healing.
You can interact with one item per turn as a Free Action, which lets you draw a weapon. Or get out a potion. But drinking one is an Action (PHB 153), so you can't use it. And remember, you have your hands full of steel, so you're going to need to drop one of your weapons in order to pull out the Potion.
What I think you should do with this round, after switching to a finesse weapon (+1 Rapier) so you can get sneak attack:
- Move 10' to engage
- Attack with the +1 Rapier, getting Sneak Attack bonus dice since your friend is engaged with the enemy
- Use the Bonus Action granted by Cunning Action to disengage
- Move 10' behind the pillar
You don't get a second attack, but it's still a heck of a round - you get to smack a foe with Sneak Attack damage and get out of combat.