Give it a shake to hear if there is any liquid inside. If it sounds empty and you can't hear any sloshing you can push the valve down with a screwdriver to empty the last bit of pressure.
Obviously do this outside and if your not 100% confident it's empty then don't attempt it.
Once you've flushed it a time or two, it's as safe as it'll ever be. That god awful stench of the stuff they add to gas will linger regardless, but that's not related to any gas being present.
You did well with it only needing the adjustable. Those valves can be a right pain to remove on occasion.
When its empty you can belt it one with a punch and knock the guts out of the valve to fill it with water.
I got a big old flashback after cutting one after filling and draining, nearly lost me eyebrows. I only ever cut them full of water now, letting it drain out as I cut with a grinder.