01.04: Combat Tracking
Multiple Actions in One Combat-Exchange? - NO
I don't know if last round's +6 stealth roll was enough to avoid whatever attack would have come against me (with advantage because of the dust) AND create an advantage I can use this turn.
Last round? Wait, you did a Stealth +6 roll last round. You mean Combat-Exchange #2? I missed that entirely. I'm going to assume you mean this round... Combat-Exchange #3.
You've got a roll in the Dice roller that says:
10:22, Today: Etzagith rolled 0 using 4 Fudge dice. Stealth (+4) to eliminate a disadvantage.
I presume we're talking about Dust in His Eyes? What you're looking for is an Overcome action. And that's an action. It will take an entire combat-exchange. It also requires some sort of mention in the in-character story.
Seriously though, on it's own, it's just an Aspect. It doesn't do anything unless it's compelled. Sure, there is a possibility that one of the bad guys might compel Dust in His Eyes, but there's just as much chance that it doesn't get compelled. If you decide that it's important enough to spend an entire combat-exchange on, then okay. It's your call.
Finally, I'm not finding any reference in the books that support splitting a dice roll to allow for two actions. Hiding, by it's very nature is Creating an Advantage. You just can't use it in the same combat-round. So... Etzagith is hiding (Stealth/Create Advantage) or attacking Hollis (Attack). Or he's wiping the dust from his eyes (Overcome). He can't do all three in one combat-exchange.
Multiple Scene-Aspects on one roll? - YES!
Result of +4, using the advantage Three-Sheets to the Wind or, if I have it, the advantage I'm way up here, now. It is allowed to use both, or are you limited to a single environmental advantage per action?
You can certainly pile on the Aspects. Free-invokes if they're there don't cost you anything. without the free-invokes, they cost you a fate-point...
I'm way up here, now
Where did this Aspect come from? I have no record of this Aspect, neither in the narrative nor in the dice roller. You did have How'd He get There, but moving from that location changed the context when you threw the blackjack... That specific Aspect is gone, now. If you want to create another similar advantage, you will need to make a roll to Create An Advantage. This is an option you can definitely take if he's now hiding... But, again... that's an Action. Is it worth the entire combat-exchange?
Do I still have my blackjack?
If you decide that I do still have my blackjack, then I'll be using blades which is +1, for a result of +5.
You threw the blackjack at the caster. It's probably on the ground now. I don't see any reason in the narrative why he couldn't pick it up on the way to Hollis...