So I'm puzzled by the wording of these features within this subclass.

Spellblade: " (3rd level) You can use an action to both make a melee weapon attack and cast an illrigger spell you know that has a casting time of one action."

Hellish Versatility: "(7th-Level) Once on each of your turns, you can cast one of your illrigger cantrips in place of one of your attacks granted by your Extra Attack feature."

From my understanding, most cantrips are, in fact, spells with a casting time of one action, making it seem like Spellblade is very similar to the bladesinger extra attack feature at 3rd level, but the Hellish Versatility feature adopts it's word choices. 

Is the implication that the level 7 feature is meant to allow you to cast a cantrip and action spell of 1st or higher on the illrigger list within the same action??

I just responded to this question elsewhere, and my reading (without the class) is:

Spellblade is a separate action from the Attack Action, and so doesn't trigger Attack Action-based abilities like Extra attack.

Which means that you can't do both SB and HV at the same time. It does seem to make HV rather pointless.

("It's badly written" is also certainly an option, probably even a likely one, but it makes it very hard to figure out what was meant to happen.)

That's.... peculiar.

Spellblade is an Invoke Hell option, akin to a Paladin's Channel Divinity. So it's only usable once per short rest