I think this might be a lot better than you seem to give it credit for - I would certainly be grabbing my copies of it. It's a seal version of Unsubstantiate. If you consider Unsubstantiate to be a worthy card, making a seal of it that has the exact same cost is questionable. This for example often comes up when people casually throw around like "Cancel seal" at which might very well be quite powerful as one of the main weaknesses of counterspells is supposed to be the "opportunity cost" they demand but being a seal completely side steps that.

Also regarding Unsub, it seems that effects that "counter" spells without explicitly using the wording "Counter" might be red-flaggable at common.

I agree that this is verra stronk. And it's not unreasonable to think that returning a spell is confusing to nwo.

(The justification for sealing at Cancel cost is that it's already overcost compared to Counterspell :)

It is a seal version of Unsubstantiate, that's true. And I agree "Seal of Cancel" is way stronger than Cancel and probably too good.

I think big fact in this card's favour is that the spell isn't countered permanently. It's not even Memory Lapsed, nor vetoed for a turn a la Reflector Mage. If the spell is cheap enough, they can just recast it immediately.

I look forward to seeing how this plays. It may be that it's much stronger than Seal of Removal, but it may not. For example, most combat tricks are cheap enough that it won't function as total protection against combat tricks.

As for common vs uncommon? Hm, yeah, I guess you might be right. The source material is very short on cards that can play the role of the common bounce spell, partly because FOME doesn't introduce new mechanics apart from Fabulosity so there's not much scope for "simple bounce spell with mechanic X" that hasn't already been done. But that's not really an excuse for uncommon-worthy effects at common.

That said... when Flicker was first printed, Wizards worried it'd be too complicated except at rare; now most sets get a Cloudshift variation at common. (Including this one: Magician’s Exit.) I'll test this out with a new player of my acquaintance who's interested and see how he finds it.

It's certainly an exciting card. I could see it contending with the likes of Remand in the older formats. Regardless of that, it might be a perfectly fine card in this limited environment.