What might be an in-between solution is using a Shelly relay (with a temp sensor attached). From what I know, they have built-in automation/scripting possibilities that you can use as failsafe, then use Homey for more elaborate logic.
I have a similar though not life threatening delicate situation and I double all the devices using two boxes from two different entreprises + a 4G box which switches on when the boxes detect an Internet failure.
Not 100% failsafe but pretty close
Depending what are your need but anything that can be made highly available. Including power, internet (if needed) and even a secondary location if you want to push it that far. It does not matter how simple or stable something is, a single piece of hardware can always fail. SmartThings now offer this option with one Hub being the main one and the second one being a backup that can automatically fail over. In my case and many other cases, this is not needed but just sharing in case this is a must for you. I also have to add that in my experience, SmartThings is not more reliable than Homey.
I have made some redundancy by combining Homey Pro and Shelly stuff (wifi âgen3â).
Their plugs can coexist in a swedish multi wall outlet (a little tight, though).
Paired into Homey with the Homey Shelly app (direct on IP-number) and added to Shelly app in parallell.
Homey Netscan app alarming if a Shelly gets offline.
Shelly tel. app connects to devices locally if you are on the same LAN and through cloud if away.
You can also configure a second WLAN in device to make it shift on failure for WLAN1 ( for ease - same LAN).
âGen3â plugs also supports local js. that might help.
I have a few Inner sp240, but they do not perfectly. Energy measurement works for a few, but not all. I also have the impression that the router functionality is not working well. Also the Zigbee performance in terms of distance between devices is, compared to other smart plugs, worse.
I have one Eve answer plug that uses Matter over Thread. Definitely better radio performance than the Inner. Could be due to Thread.
I bought recently an Aqara Duo Relay Module T2 and are pretty happy with it!
All devices locally connected to Homey. You can connect a temperature sensor to the Aqara module. Havenât tried it as I am missing such a setup. This could give you the redundancy you need
Sounds like an additional spec?
(Although I doubt if many user have experience with thes temp 
)
What minimum temperatures do you expect to have at sockets location?.
These are pretty cheap in Sweden now and I use (indoors) and seems to meet most of your initial requirements at least. Have you considered them?
I have not tried myself. But what about ikea plug? It use zigbee and works without internet i think. But not sure. And is low cost
Energy reporting for this plug is broken unless you update it to its latest firmware (which isnât possible with Homey). My local IKEA doesnât stock it anymore, probably because of this issue.
Its probably gonna be a bit warmer at the socket location, but I expect temperatures below -30 to hit that location, maybe not very often but it will happen for sure.
You think temperature is something that could be an issue 
Oh, never seen those! Have you used them a long time and do they seem stable over time with Homey?
Also, do you know if they properly create a zigbee mesh when using with Homey Pro 2023? (Able to reach very long distances by chaining them)
I bough one or two last winter to reduce space in outlets and itâs been working fine - I havenât had any issues with my older Hue plugs either except size, so maybe Iâm not sensitive but Iâve never had to repair the connection or such. So for me itâs been connect and forget.
They are (like most plugs) Zigbee routers yes. I noticed the Norwegian page has more details and tech specs: Namron Zigbee Smart Plug 16A hvit | ElektroimportĂžren AS
Yeah, but Homey doesnât seem to always be able to correctly chain multiple routers (router connecting to another router rather than everyone connecting straight to the homey), I bought some Zigbee 3.0 plugs but they only worked when in close proximity with the Homey, not when only in proximity of another plug.
Its not an absolute must have but it would be nice to be able to build a stronger zigbee mesh if I do want to have some end device far away from the Homey at some point.
I donât think itâs possible to visualise or list the Zigbee network in dev tools on the 2023 model so not sure how/if anyone can confirm how they actually are connected.
That said, I have tested a plug in the garden shed some 20m away from my house, and the Homey is in the middle of the house and that worked just fine so I would guess they formed a network and not just hub and spoke.
Sounds promising, though 20m might still be within range to make a direct connection to the Homey.
I had a situation with a plug indoors about 5m away from the Homey, one outdoors at 20m (through a thick brick wall), and a third one outside at 30m (all on the same line of sight to the Homey). Both the 5m and 20m plugs worked just fine, but the one at 30m just wouldnât be part of the network 
Unfortunately, as you said, its not possible to visualize the zigbee network so its really hard to debug these things.
the differential temperature coefficient of expansion between the parts-soldered-to-the-circuit-board and the circuit board can cause large stresses.
With temperature differences that largeâŠ. Might cause cracks
Popp outdoor plugs âpoppedâ into my head as candidate, but operating temperature range is -10°C to +45°C with
humidity: 10 to 95%
And theyâre z-wave plugs.
About zigbee:
Zigbee is not plug & play, and not everything is compatible (when used with Homey).
For a really good zigbee controller I can recommend zigbee2mqtt for 100%
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Iâm really a noob at this stuff, isnât homey already a controller?
When googling on zigbee2mqtt it seems to be a software, how would that work with homey?
Looking at the community app for Homey called Zigbee2MQTT there is a line about already needing to run Zigbee2MQTT on a Raspberry PI or similar.
Seems a bit complicated 
Yes, Homey is a zigbee controller, but not the best, and very limited compared to monitoring and self-checking, and pairing multiple devices on the fly.
Thereâs also weird compatibility issues between certain brands, which donât occur with Z2M.
Indeed Z2M needs to run at separate hardware, like an Rpi (Raspberry Pi) with a Sonoff Zigbee dongle, and it communicates with Homey per MQTT (which uses wifi)
But I can imagine itâs all too complicated at this time.
I can recommend to run Z2M as Home Assistant add-on, hereâs a âfrom A to Zâ tutorial I wrote
I can confirm that behaviour with the INNR SP240 plugs. It would be really great to see if if routers are chained properly. Maybe Athom can also increase the performance of their Zigbee controller somehow.
