Is it permissible to ascertain that a wound is free of congealed blood by placing a wet tissue on it, and to be content with not seeing blood on the tissue for ablution without removing this congealed blood, in order to avoid causing it to flow and consequently having to cover the wound with a medical plaster that prevents water from reaching it? And what is the ruling on previous prayers performed in this manner?

Ablution (Wudu') is not valid if there is a barrier preventing water from reaching the skin. However, some scholars, including Sheikh al-Islam Ibn Taymiyyah, held the view that Wudu' is valid with a minor barrier.

If you are certain that a barrier exists, you must remove it before performing Wudu'. If you are in doubt about its existence, the default assumption is that it does not exist.

You are not required to repeat past prayers unless you are certain that a water-impermeable barrier was present.