Hi MoodyPond, thanks for this question. This is difficult to translate, but in this context there is a circumlocutive element to both ～くもない and ～くはない.
Here, 見えなくもない means 'it's not that she doesn't look like...', which is less direct than 'she looks like'. In the case of 決して若くはない, the は kind of softens the blow; if you were to simply say 決して若くない it would be very direct and perhaps rude. In short, the も and は, at least in the example of this excerpt, are ways of avoiding directness.
(However, for example in the case of 安くもなければ高くもない, the も + negative simply means 'neither' and 'nor', as in 'neither cheap nor expensive'.)
What is the difference between 〜くない, くもない, and くはない? Never could figure this out...
Hi MoodyPond, thanks for this question. This is difficult to translate, but in this context there is a circumlocutive element to both ～くもない and ～くはない.
Here, 見えなくもない means 'it's not that she doesn't look like...', which is less direct than 'she looks like'. In the case of 決して若くはない, the は kind of softens the blow; if you were to simply say 決して若くない it would be very direct and perhaps rude. In short, the も and は, at least in the example of this excerpt, are ways of avoiding directness.
(However, for example in the case of 安くもなければ高くもない, the も + negative simply means 'neither' and 'nor', as in 'neither cheap nor expensive'.)
Thank you!