I have mostly Billys and after research, planning, diagraming, and measuring I went with them to fit my needs since most of my collection is 1:6.
Click on the below collection link to see them.
Yes they do have adjustable shelves, but the middle shelf is not adjustable so really you get 4 total if you are going to display 1:6 figures. They also have a few door options but yes you will always have the split down the middle if you get the 31" wide case. I still have not got doors for two of my cases (one is not pictured yet). But the thin ones you can see I mostly only have 1 figure per shelf.
I was able to get the 14" deep case (which have my Marvel movie/HT figs) when they were still around and then about a year later when I went to get another one, I could not find them. I emailed Ikea and unfortunately they were discontinued!!!
Just one note, even with doors there is about 1/4" to 1/8" gap between the door and case, so over time you still get dust in the case. I am planning on putting some weather stripping to solve this, but have not done it yet.
One last note, the cases and doors are slowly starting to go up in price. Back when I got them (few years ago) they were $59 for each case, now they are $79 for a case, and the doors went from $45 to $55...
(I hate inflation!)
But love them Billys!