Hi

The Port is one solution. However you could purchase another Era 100 and use its line-in as an actual speaker. Once the music is following through the Era 100 you would group it to your Beam2 to have the sound play through that entire system.

You could purchase two Era 100’s to have a dedicated stereo pair for the TT when you don’t want to group to the Beam2.

No need to group anything. The Era100’s Line-In would be directly playable on any device.

Ideally I would like to keep Beam + 2x100 for TT and Surround (and seamlessly switch between) without having to buy extra speakers to do a separate job. The living space is fairly small as is, so adding more era 100's feels over spec'd.

Sounds like the Port is the solution all by it being quite pricey for the single task it provides.

But I'm guessing the delay from Port to Era 100's and Beam will be a lot better then the current line-in to Era and out, is that the case?

All Line-Ins are delayed at least 70 ms in order to buffer the input for multi-room transmission. 

To make sure it’s clear, you can’t have music playing on just your 2 surround speakers (Era 100s), whether that be from the turntable or any other source, and not on the Beam, while those 2 speakers are configured as surround speakers.  The solution of using a Port or another Era 100 connected to the turntable would allow you to play the turntable on your Beam+surrounds room...not on the surrounds only.

And as pointed out, if you're only interested in the line in functionality of the Port, rather than line in and line out, then a separate Era 100 is a cheaper option.

If the TV’s connected to the Beam via optical (using the Sonos optical-HDMI adapter) it’s possible to feed the turntable directly into the Beam via (a) an analog-digital converter and (b) an optical switch.