there are still seats free but Eurostar charge for them the full price.

With good luck maybe one or two cancel their Eurail Eurostar reservation but i doubt it will be 4 seats on the same train.

Plan B. take a train to the South Coast and take a Ferry to France and travel on or check the fares of Bus operators on the route.

Agreed, but point of buying a Global Eurail pass is to not have to pay for full fare. Also, several reviews indicate that travel from London to Paris via ferry is now +10 hour journey. Not something I want to do with kids.
 

My question is still open. There are still seats available on the desired trains, but the fare is not currently available for Eurail fares. If those seats are still unreserved on the day of travel, will the ticket office open them up? Does anyone have experience with this? Is it worth going to the train station if there’s still open seats available?

No, they won't. Simply forget this for 4 people.

€* has NO ticket office-its all online. There are rather small QUOta for passholders as their income is far less as from full fare payers. THis is not the USA where bisnis prevails.

we are in the same situation - 2 seniors bought global pass to land in London, take the chunnel and travel through France. I am wondering if we exchange our Global pass for 1 county pass will they refund the difference. I know there is a 15 pound charge for the exchange.

They sell the free seats only for the Full price when the Railpassholders limit is reached they wont add extra seats for them

I still see these options for London - Lille 4 persons in 1st class:

On the direct London - Paris, the first available seats are on 2 June 18:01.

I still see these options for London - Lille 4 persons in 1st class:

On the direct London - Paris, the first available seats are on 2 June 18:01.

Thank you. I booked a train from Lille to London, instead of Paris to London.