How much did Howard owe? And "the bad guys"? Ending?

can't remember they exact amount but he could have paid him off multiple in the movie, i remember that

Not sure how much was owed. Probably around 100K. I think the implication is that Garnett's 165K or so was enough to cover it.
I agree that it seemed earlier like Arno was the boss. But in the final scene it seemed that Arney was an underling.
My sense was that they were ready to leave the store, and then probably kill Howard later. Then the leader got mad for being locked up (understandable) and that's why he robbed the store and shot Howard. But I did find this robbing aspect very odd. There seemed to be tons of jewelry in the store. Why would they not just insist that Howard give them a ton of jewelry as collateral? (I mean -- they were just dangling him out the window to get the cash...)

My take is that he took a loan of 100K from Arno 17(?) months ago to buy the opal. Arno hired Phil and Nico to get the money back. In the end of the movie Phil felt so humiliated by being captured in that room and probably didn't think he would see the money even though Howard won the bet (since Howard got a serious gambling addiction) - so he shot him, and then Arno when he, of course, started questioning Phil's action.

I was just thinking the same thing when they started robbing the store... I was like wtf?

I think he owed around 100k. I don't think Arno was a "leader" as such, nor was there any kind of organisation.

I think Arno was just a successful, independent loanshark, who hired these goons to put the squeeze on Howard to get his money back.

But Howard put the goons through so much shit over the course of the film that they no longer cared about their contract with Arno, and especially after being locked in the room their beef with Howard became personal.

Goon #1 was so pissed off and humiliated that he shot Howard on impulse, and since Howard was now dead they thought they might as well rob the place too and try and profit from it. I honestly think the robbery was an afterthought, but judging by how he commanded the robbery and focused on the cctv drives etc, it wasn't his first aggravated robbery.

I never saw Arno as a boss, I think he just hired thugs to do what he couldn't do himself and then totally lost control of them. I think part of the lesson here is that Howard and Arno were BOTH in over their heads.