even though every sign [of the truth] should come within their ken - until they behold the grievous suffering [that awaits them in the life to come].

Even if every Sign was brought unto them,- until they see (for themselves) the penalty grievous.

Though every token come unto them, till they see the painful doom.

Though every sign should come to them, till they see the painful chastisement.

Even if every sign should come to them, until they see the painful punishment.