Hi all - here's a homemade patch for the Eco issue. Please note I'm not affiliated with the developer - just a user hacking his way to a fix.
NOTE! This is not a complete fix! Just a quick homemade patch for getting rid of the annoying errors in the Event Log.
To make this homemade patch, you will need to be comfortable with digging around in the XML files that configure the NEST Home plugin. That said, it's pretty straightforward.
First, you need to locate the plugin on the computer running your Indigo Server. These instructions assume you are running macOS Sierra and Indigo 7; the instructions should also work for recent older macOS and Indigo versions, just be sure to go to the right directory for the version of Indigo you are running.
Open TextEdit, go to TextEdit->Preferences, and disable Smart Quotes
On your hard drive on your computer running your Indigo Server, navigate to /Library/Application Support/Perceptive Automation/Indigo 7/Plugins
Right-click the NEST Home plugin and select Show Package Contents
Within the package contents, navigate to Contents/Server Plugin
You are going to be editing the Devices.xml file. Note that if you just double-click it, by default this will open it in Safari, which is not useful for editing. Instead, you will right-click the file and Open With->TextEdit. It's important you've turned off Smart Quotes because otherwise TextEdit will annoyingly change the " character to a smart quote, which Indigo can't parse. You could also simply Open With your favorite code editor (Xcode, TextWrangler, etc).
In Devices.xml, search for:
<State id="hvac_mode">
And to the list of options under that, add:
<Option value="eco">Eco</Option>
This will get rid of the annoying "cannot update device state for key hvac_mode" errors in your log.
Finally, open up the Indigo client, and select from the menu: Plugins->NEST Home->Reload and check your Event Log for any errors (due to typos, annoying smart quotes, etc)
That's it! Lather, rinse, repeat if you have additional homes you're controlling with the NEST Home 2 and NEST Home 3 editions of the plugin.