if I add my Sonoff to the eWeLink app (as if i didn’t have iHost).
Then open Sonoff iHost web interface
Add the devices from iHost (either via Zigbee or LAN).
Do not unlink the device from eWeLink Cloud.
then Can I access them remotely and while they are on local they still work if internet is off?
That’s a one-direction link, most devices on the eWeLink cloud can be synced to iHost. But devices connected with iHost via Zigbee can not feed back the eWeLink app.
The iHost interface can be accessible via Tailscale or alternatives, you can also control the devices on Matter platforms like Apple Home