It would still restore MP at the same rate that ED does. I don't get why you missed that. The only reason for it to affect healing potency at all is to give it a minor boost beyond simply restoring MP, without affecting SCH's shield potency at all. SCH already has a lot of tools to improve their shields. They don't need another. Their GCD heals, however? They could use the boost sometimes, particularly in ARR when the 150 potency heal on Succor and WD are all you have for topping off a dumb group. And there are still level cap situations where you have to toss out Physicks. Alliance Roulette's a good example. You can't lustrate a different party member. And the MP boost would help pad through the loss of using Raise repeatedly more than the Lustrate would, while the healing boost would help make up for spending Aetherflow in that manner. It's not good by any means, but it gives SCH slightly more reach in their worst area, without touching on their best. Most likely you'll simply use it on a Ruin II to move/weave later regardless. That's plenty. It doesn't need to 'beat Lustrate' in potency to be worth using when it already pays for 80% of an Adlo and a third of a res. It's something minor that would be appreciated in Alliance roulette and ARR more than anything.
DPS wise, however, it's still a significant nerf, because it only works on Ruin spells. Not Miasma II, not Broil II. Ruin II is the only DPS spell left on SCH's kit that it would work on. And as a result it is a 50 potency nerf per ED used in the best case scenario, which is one where you're specifically out of melee range or need to move for a long consecutive period of time regardless of whether you're in melee range or not. It's a complete loss if you were using Miasma II to weave them in as much as possible, because now you're cutting into your Miasma II usage in order to weave it in at all, forcing you to instead use Ruin II. You'll rarely weave Miasma II in at all this way, basically only when you had to use the potency of a prior SCH-Focus on a GCD heal instead, already have Bio II rolling, and have to move/weave, and are in melee-range, which is pretty restrictive as it is. So yes, it addresses the problem on all fronts. The only concern you might still have is how to spend all of that MP you've effectively gained from cutting out Miasma II, which...hey, you could use it on your expensive healing spells. Or just keep spamming Broil II. Either way, it's less damage compared to what SCH can currently do. How much simply varies from fight to fight.
This is an ED replacement, with nearly the same functionality ED currently has for SMN's specific interactions. The rest of both kits are completely untouched. The only restriction it would need is for the Ruin stacks to only exist inside combat to prevent opener shenanigans. Hell the SMN-Focus trait could trade away the MP for the extra damage just to preserve their current MP values. Regarding Aethertrail, it would proc when you use SMN-Focus, like any Aetherflow ability currently does. It does not gain Aethertrail stacks when you expend the Ruin stacks it gives. All of the current SMN aetherflow abilities require an enemy to be present to work at all though, which cuts into your rotation during jump phases. This guarantees you can get DWT and Bahamut rolling on time, no matter what the fight's like, and lets you go for a Fester/SMN-Focus/Fester opener, which has the same timings as the Fester/Painflare/Fester opener but doesn't lose potential potency to speed it up. It actually lets you squeeze more into the current DWT. Bahamut is the only exception due to the way it interacts with your oGCDs, requiring a target to proc Wyrmwave, but it doesn't change up the core rotation much. You'd use it twice in your non-bahamut Aetherflow window (SMN-Focus/Fester/SMN-Focus) to get to DWT faster and use 3-4 stacks inside DWT for a minor boost of 30-40 potency every 2 minutes. If the Aetherflow lockouts were removed you'd be repeating the opener inside DWT anyways since Fester/SMN-Focus/Fester fits in cleaner than Triple Fester does, which is still 90 potency every 2 minutes over the current rotation for comparison.
The reason for the divergence from the base ability of Focus into two distinct Job-specific abilities is simply because SCH and SMN have different needs that ED doesn't help with, but those needs still share some commonalities. It's a slot worth replacing on both jobs, and functionally what it does is still similar on both, but tweaked to deal with their respective problems. SMN needs an ability to use Aetherflow on during downtime, and specifically needs it to compete with Fester to be worthwhile to use at all. SCH needs the MP more than it needs the potency, and I address the latter concerns above. Focus is specifically so Arcanist has something to do at a low level and ties it all together.