一層 is a more common and straightforward adverb meaning 'even more' or 'much more', while ひとしお carries a nuance of heightened feeling due to circumstances.
なおさら means 'all the more' or 'still more', often used in logical or causal contexts, whereas ひとしお is more emotional and situational.
Etymology
Derived from 一 (ひと, 'one') and 入 (しお, an old counter for dyeing or soaking), originally referring to one more dip in dye to deepen the color. The modern adverbial sense developed from the idea of adding intensity.
