If I understood Ark appropriately, pool transactions (aka on chain transactions) are linked, as within the output of pool transaction A is used as enter on pool transaction A+1.

In that case, how can it work if there’s this mempool coverage that limits the variety of ascendants/descendants on unconfirmed transactions to 25?

With pool transactions each 5 seconds that might imply a sequence of 120 (on common) unconfirmed transactions on the mempool.