I am guessing that the stability issue comes from oxidation of...

I am guessing that the stability issue comes from oxidation of the Si to SiOx in situ. There is probably a fix for this that will involve coating the Si with some oxygen impermeable coating. This might be part of the stress issue too.

The residue will probably come from the etching agent that is used to remove intermediate layers of scaffolding/support. Some presumably gets left behind as gunge. Again I am guessing from their comments recently that the solution will be a relatively simple rinsing process adjustment.

I think the 6-18 months mentioned was the option to change the deposition process that creates the layers. Instead they changed the membrane and that led to a single optimal geometry and fabrication process. That was discussed in earlier reports. The result of that change is that they already did it and incorporated it.