Should one rub and clean hand cracks before ablution if there is no substance preventing water from reaching the skin? And is what enters these cracks considered a barrier? And is the ablution considered valid if the color of the cracks changes by touching ash without removing the color? And is it obligatory to repeat the ablution in case of doubt about the presence of a barrier before touching the ash?

Water must reach inside the cracks during ablution because they are part of the outer body. An impediment that prevents water from reaching is something that has a physical mass, not merely a color. Caution in matters of Sharia (definition) is good as long as it does not lead to contradicting the Sunnah (definition) or to obsessive doubts. If a matter befalls a Muslim and he cannot find a mufti (definition), he must fear Allah and seek the truth to the best of his ability, for Allah Almighty has set signs for the truth, and a sound innate disposition (fitra) inclines towards it.