I agree about separating the top/skirt on the black one, the weight of the fabric won't do the top any good in the long run, though you'll have to very carefully steam out the marks that the stitching will leave. Hold the effected area close to a boiling kettle (probably best patch test it first on a discreet bit), the, brush the pile with an old soft toothbrush. Brush it again once it's dry, and the marks should be gone.
That paler dress...ohhh, it's got some problems, that's for sure...
From the pic of the underarm damage I'd say it probably is silk. Sweat rots old silk in a really alarming way, and ballgowns/theatrical costumes seem especially prone to it, as people get really sticky in them and they're hard to clean regularly. Also, some old silk just goes crumbly, it dries and cracks. Taffeta seems to really be cursed by it, and I've not found any way of dealing with it that works anything more than temporarily. I had a lovely old black and white regency stripe gown that completely disintegrated (it literally turned to dust, though it was well on the way before I owned it) because of this, all I have left of it is a few yards of the most amazing Victorian hand knotted ball fringing.
That shoulder damage, is it an actual tear, or has the stitching just given up on a seam?
TBH, the best thing with that one is keep running emergency repairs on it but resign yourself to it being in a genteel state of terminal decay. Just seems to be the fate of a (thankfully small) number of old silk gowns.