Eurostar

You are aware of the current disruption on the Oxford-Paddington rail line. A bridge is out south of Oxford and there is replacement bus service from Oxford to Didcot Parkway. This will go at least until June 10.

I do not know about the detour…..if our ticket has been bought what do we do?

https://www.nationalrail.co.uk/service_disruptions/321909.aspx#GWR

Would you be able to change your hotel to one near St Pancras for the night before your Eurostar reservation and turn in your a day early in Oxford?

Eurostar suggests you arrive 90 minutes before the departure time. Gates close 30 minutes prior to departure: https://www.eurostar.com/us-en/travel-info/your-trip/check-in

Re- the Oxford to London journey. Allow an extra hour to your journey, although you shouldn't need that much. There is a well organised bus shuttle from Oxford Station to Didcot, for the train to Paddington. Or you can take the Chiltern rail route from Oxford to London Marylebone, which is on the same tube line to St Pancras, a few stops closer than Paddington.
No matter what ticket you have, it is valid on the Chiltern route, and if an advance ticket so long as you are not hours out, no one is bothered about the time on it.
At St Pancras as long as you are there 90 minutes before departure then if you miss the train that is Eurostars problem, not yours. They either delay the train or transfer you to The next one.
The whole situation is going to be made even worse if people start arriving even earlier.
It is no different to airport delays. Just arrive within the specified minimum times.
It is not clear what caused the unusual situation this weekend.

PS- your much bigger problem that day is that June 3 is a scheduled rail strike day. We won't know until next week, when train plans are due for the first ASLEF strike in May, what service level can be provided.
However it may be necessary to take the Oxford Tube inter city coach service from Oxford to London Victoria, then the Victoria Line to St Pancras.
Allow 2 hours for the coach journey, but it is usually faster.
So that would be a 9 am departure from Oxford Coach Station on what is a very frequent and very comfortable service.
Arrive Victoria 1100 or sooner, 10 minute transfer to the underground (actually less), 10 minute train journey. You are strolling unhurried onto St Pancras by 1130.

alternatively maybe return the car at Heathrow if you can, and then easy tube to Kings Cross St Pancras...... a possibility?

do we need to allow more time to stand in a line before boarding the Eurostar?

Jane, you're not allowing time to stand in a pointless line as a mindlessly-obedient tourist. You are allowing time for mandatory Immigration and Customs formalities that you may have mis-imagined to be conducted in the country of arrival. Even before Brexit, Eurostar required at least 30-45 minutes early arrival. And they have DENIED BOARDING to people who arrive too late, even if before train departure. Use the Search box here if you don't believe me.

You have four travel "seats" on that June day. And that's if you can walk to your hotel in Paris. That is a tedious and tough travel day. Even when rail travel is reliable, it is not perfect. And taxis in a major city of the world are always subject to traffic delays, as well as delays in the taxi line at major rail stations. I've seen some depressing lines for both the Metro and taxis at Gare du Nord in Paris, too.

https://community.ricksteves.com/travel-forum/england/current-eurostar-experience

A little further explanation: You go through French border inspections at Pancras as well as security measures. Once in Paris, simply walk out of the station. But do allow as big a margin of time as you can for the often-crowded St. Pancras.