Keen support is simply suggesting that I factory-reset this vent that's stuck toggling back and forth. Also they reiterated that my particular vents are out of stock still--so no replacements for me.
My main supply plenum splits into 3 trunk ducts... one for each level of the house. This is not common, but something I requested when we had the house built over 10 years ago. I recently added motorized Honeywell dampers to each duct and control them with custom rules based on where I want air to go. A Zooz ZEN16 is used for the "smarts." Everything is hardwired and powered by a 24VAC transformer. It has worked great this winter and I'm looking forward to seeing how the summer season goes.
If you have access to your ducts, you should be able to do something similar. There is also a great HVAC zoning app that can help. Includes options for dampers, booster fans, etc. The documentation is great and has some really helpful info for avoiding things like freezing your AC coils and the like. I really wanted to use it but my ecobee thermostat doesn't play nice with the app.
You're absolutely correct that precise control of small zones is a challenge, especially with residential equipment. Even larger commercial systems can struggle with zones that are too small or where loads/setpoints vary widely. In some cases designers will use a mini splits (or their variable-volume commercial equivalents) to do what @Sakman mentioned. But if you understand the capabilities and limits of your equipment, it is certainly possible to optimize it and get better performance than your typical residential contractor can do for you.
So I have 6 Keen vents in the house and I have a battery life issue with only one. It seems to make constant updates to my ST hub where the others do not. I think it's the model version that's defective.
These models have no battery issue:
SV01-612-MP-1.0
SV01-610-MP-1.1
SV01-610-MP-1.0
This one sucks:
SV02-610-MP-1.3
I'll have to check what I have. They're all worthless.
How long does your 'bad' one last on a fresh set? How often do they move?
So I just removed the offending Vent from my ST Hub, held down the button on the vent hardware for a bit of time until the lights did some flashing, etc. (Sorry, I didn't pay attention to the color(s), etc.) I assume it was a sort of reset. Then I re-paired it with my ST hub. This time, the activity has calmed down. No new events were updated until I adjusted the vent % opening almost 30 minutes later. I think the batteries should last longer now. Prior to this, I was getting several event updates every minute.
I've been using lithium rechargeables on the unit. I practically "never" adjust this one. Every time I happened to want to adjust it via Alexa, I notice it's offline and the batteries were drained. If I had to guess, the batteries may last a week or two...at the most. The event history on the device showed updates hitting the ST hub a couple every minute. As mentioned in my other post, after unpairing, hardware resetting, and the re-pairing, the events have completely calmed down to only occurring when I interact with it. Also of note: I observed the battery issue whether the device was hopping straight to the hub or through my Ikea plug.
I just noticed that you are using the ST hub. That's kind of the whole thing. Hubitat has problems with various devices.
Examples please?
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In my specific case I had issues with the Aeon Labs multisensors having massive battery drain, which there are endless threads on,the keen vents, and I never could get my Schlage FE599NX to pair to my c7. It's the only reason my c5 is still in service. Hubitat is great, but there are a few quirks, is all I'm saying.
FWIW, this lock has issues with other 700-series controllers.
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I agree with the opinion above.
My A/C guy told me that closing vents is not a good idea for central air conditioning. Actually, several A/C technicians over the years have told me this.
The best way for more granular control is a mini split ductless system.
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As a "blanket statement" that is false (that's why they make vents closable...). And the techs know that (they aren't dumb), but it is an easier/safer answer.
Whether it is harmful depends if air handler has a high pressure spillback/overpressure vent, depends on how many vents are closed vs total, do you have a variable speed blower. etc.
There are definitely circumstances where it is true, though - too many vents closed can cause too much back pressure can wreck/harm the blower, too little air flow over the coil can be harmful, etc.
So I get why some techs just say "don't close them", as the real technical answer is complicated and "it depends", which is too hard/nuanced to explain to most customers.
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I think the reason most say this is that the majority of single zone HVAC systems are very basic, especially the units that are a prime target for Keen vent customers.
No argument there.
I'll be moving to Hubitat soon, as ST is depreciating their groovy platform and many of my custom device handlers will likely not convert to an existing edge driver.
However, there are discussions on the ST community about the battery life issue as well, so this might be a hub agnostic problem. I only experience the battery issue with a vent I installed about two months ago after my other vents had been working fine for several months.
I guess the vents will be the first devices I try moving and I'll be sure to update on the results either way. It'll probably be around November.
Maybe they never had a significant other that likes to keep raising the thermostat to 80 because they're cold even though the house is set at 76, and the room they're in is 78. Those vents may have saved my marriage because my wife can regulate the cold air being pumped into the room she is occupying without having to grab a ladder and one of those "pickup & reach" robot arms.
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That's what space heaters and warm clothes are for. 
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What is the relative humidity level inside your house? If it is very low, say 40% at 76F, You might consider raising RH to be about 50% at 76F. It will make 76F feel much warmer on human skin.
Basically, we detect dew-point, not temperature. 40% RH at 76F is a dew-point of 49F, whereas 50% at 76F would be a dew-point of 56F. 49F would feel cold. 56F would be very comfortable.
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An alternative might be a vent booster fan..
https://www.amazon.com/vent-booster-fan/s?k=vent+booster+fan
You could control this via a smart plug or outlet. I guess the concern would be too low pressure maybe?
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