The reason why you could not find them is that the construction requires the subjonctive ("je sache").

Pour autant que je sache,

Pour autant que je me souvienne,

Or if you remember something ("se souvenir de qqch"):

Pour autant que je m’en souvienne,

For "comprendre": I cannot fault the phrase "pour autant que je comprenne", but it does not sound colloquial. I would definitely say:

Dans ma compréhension,

Si j’ai bien compris,

Si je comprends bien,

In spoken language, however, you could say:

Pour ce que j’en sais,

Pour ce que je m’en souviens,

Pour ce que j’en comprends,

In my own experience, the first is frequent, the last two are a little more clunky.

I do not see any deep truth here, it is just a matter of common usage.