The Demonomicon Tome might be what's needed to give summoning a new lease on life.

[Deathtongue]

About Summon Shadow Serpent: the key is that the melee basic attack allowed by the Demonomicon is a free action, you can only command the Shadow Serpent to take actions listed in its stat block, and most importantly that all summons can only take actions by drawing from the summoner's action pool. If the melee basic attack allowed by the Demonomicon was not a free action, just something the summon could now do (like Instinctive Actions) it'd be ambiguous, but since it's a free action it's not ambiguous.

You make an interesting point about the summoned creature being asked to do things that it doesn't have statistics for, but that's a general divide-by-zero error with all summons, not with this specific trick. This problem does come up with lifting and with powers that key off of a target's game statistics, but summoned monster attacks, whether basic or otherwise, don't use the creature's statistics. But if divide by zero sitautions bother you in general, you should ban summoning altogether, because it's a problem whether or not you use the Demonomicon. But the Demonomicon doesn't require you to divide by zero.

So "If a summoning power allows the summoned creature to attack" -- you are imputing "by making the creature exist, the summoning power is what allows the summoned creature to make a basic attack"?

There are some summons that don't allow the summoner to make an attack at all, such as Summon Shadow Serpent. You can only command the Summon Shadow Serpent to take actions listed in his stat block, and they don't have an attack listed as one of their actions. Therefore you can't command them to attack. This might prove to be a problem if some effect allows them to take an attack without using up an action (not even a free action) but the basis attack form the Demonomicon is a basic attack so it's a hard blocker.

But without such a restriction, yes, by making the creature exist the summoning power is what allows the summoned creature to make a basic attack so long as the summoner spends the appropriate action. A summon is a creature and all creatures can use basic attacks. However, the creatures' statistics then become pointless for making a basic attack because the text of summoning powers replace a summoned creatures' statistics with the summoner's. Since the Summon Hammerfist Crusher and the Summon Iron Cohort have the Implement keyword, the summoner making the attack roll for the summoner's basic attack can use implement properties on the basic attack roll.