Some preliminary definitions might be in order before identifying whether they're "similes, metaphors, or something else":
A figure of speech involving the comparison of one thing with another
thing of a different kind, used to make a description more emphatic or
vivid (e.g. as brave as a lion).
A figure of speech in which a word or phrase is applied to an object
or action to which it is not literally applicable.
To put it simply, a simile describes one thing as a likeness to another, while a metaphor describes one thing as being another. Someone might liken me to a mouse (simile), but if I say I'm a lion (when in fact I'm a human), that's a metaphor.
Turning to the two examples from 1984...
Winston's entrails seemed to have turned into ice.
This is a simile, not a metaphor. Seem means "Give the impression of being something or having a particular quality" – hence in the given context, the sensation relating to Winston's entrails is being compared or likened to them turning to ice. In no sense does the passage suggest his entrails were actually turning to ice.
The smear of rouge that was still on each cheekbone stood out sharply,
almost as though unconnected with the skin beneath.
If the sentence said the rouge was actually unconnected to the skin, it would be metaphorical, since such a thing is not normally possible. But the use of "almost as though" indicates that the rouge being unconnected with the skin is not a literal statement but rather an impression. The rouge is on the skin, but the way it stands out makes it look like it's above the skin. It's probably pushing the boundaries to describe this as a simile; I think it's simply descriptive prose.
Finally, the question asks:
But it is metaphorically(?) saying she jumped out of her skin - is it?
The expression "jumped out of her skin" is certainly a metaphor, but you're using this expression yourself - it's not within the text. I think it's quite a stretch to suggest Orwell was intending any connection with the metaphor; instead, he's merely accurately describing her appearance. The sudden shock has drained the blood from Julia's face - "Her face had turned a milky yellow" - leaving the rouge highlighted against her pale skin. The effect is so stark that it's as if the rouge is no longer connected to the skin. No metaphor here, just vivid description.