First thing, your pass, please read/watch this and make sure you are using it correctly, you can de-activate it if you have set it up for the wrong day but now that you have applied it to a device you cannot put it on another without assistance from eurail customer services so bear that in mind if you need to change your phone.
On the general usage, yes once you either have reservations or are travelling on trains that do not require them all you have to do is add each train to your pass. there is no advantage in doing this far in advance and you change these as much as you like.
For the Eurostar trips, from/to London I assume you have bought the correct eurail passholder reservations appx €30 each?
You say you bought tickets for Nightjet separately, what have you bought and from where?
All nightjet trains in all accomodation (seats, couchettes, sleepers) can be booked for passholders at reduced rates.
Swiss trains: these do not require reservation, just add to pass and hop on.
You will use 1 travel day for any train boarded between 00.01 and 23.59. A train that continues past midnight does not use the next day on the pass once you boarded before midnight. Any further train taken the following day will require another pass day.
The TGV Lyria Switzerland - Paris are very expensive for passholders, especially 1st class, with limited spaces given to passholders. Unless you are very set on using these trains the alternative recommended here is to use reservation free Swiss/French trains to a French station where you can book a TGV seat for much less. There are a number of routes for this but from the Basel area connecting to TGV in Mulhouse is usually the best option.
Internal French TGV can be booked here https://travel.b-europe.com/eurail-ge/en/booking-tgv#TravelWish using pass cover number from here
London - Oxford, as with Switzerland, internal trains in UK can be boarded without reservation. There are 2 separate routes to/from Oxford with frequent direct trains from London Paddington or London Marylebone stations.
Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days.
Planning
The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries.
Reservations
The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new topic, and you will get advice.
Please note that Interrail/Eurail charges an extra fee of 2 EUR per person and train in addition to the fee for the seat reservation.
Activation of pass
During the activation process, when you choose the start day of the validity of the pass, the first day of the validity period is automatically made a travel day, even if you don't enter a journey, the advice is therefore not to activate the pass before the first travel day as you only can deactivate the pass before 00.00 on the day the validity starts. If your travel plans change in the last moment you will loose travel days if you have activated the pass in advance.
It can be wise to make a test and activate the pass with a start date well in the future and then deactivate the pass immediately, just to see that everything works.
Activation of travel day
The advice from the experienced travellers in the community is also never to activate a travel day, that is connect a journey to your pass and create the ticket (QR code), until just before boarding the train, otherwise you might loose a travel day if your travel plans change in a late stage You can't delete a travel day in the past. A travel day can only be deleted until 23.59 CET the day before the travel day
You use your Eurostar reservations (not your interrail Qr-code) to open the ticket barriers in the respective terminals. Then you proceed to the security and passport checks. That increased a bit in time since Brexit, so always be well in advance at the Eurostar terminal - 45 min to an hour- before your Eurostar train leaves.
If you get checked in the train, you show your interrail/ eurail QR-code, together with the reservation.
It works a bit like a plane, thatewhy I don't really like Eurostar. But still, the fastest way to get to the UK.
Exiting the Eurostar does not havy any extra hassle.
You might want to exchange the full fare tickets for the nightjet and buy discounted reservations with your pass. (But check first if tickets are still available, Nightjets are very popular!) You can find a guide in the reservation-topic, accessible via the link @AnnaB provided.
For the rest, no need to stress. You're well prepared. Just one more tip: install the app or check the journey planners on the website of the operator of your trains a bit before your departure,for the latest rail time information. You never know if there are works or issues that cause delays or devations. The Rail planner app is perfect for theoretical planning in advance, but not for real time.
It works a bit like a plane, thatewhy I don't really like Eurostar. But still, the fastest way to get to the UK.
Just a small point - Eurostar is the only way to travel to/from the UK with a pass.
The only alternatives are flights or ferries/car through tunnel - until, of course, they develop a Star Trek transporter.
OOI I like one way trips with E* one way and flight out or back. It also means I can do at least 2 journeys per pass, so a 10 in 2 months works well (for Interrail users of course).
The Rail planner app is perfect for theoretical planning in advance, but not for real time.
The rail planner app is too unreliable for any planning.
Thank you all for the replies!!
Okay so yes I have activated my pass because I thought I had to so that I could get reservations for the Eurostar. If I deactivate my pass will I still be able to keep my reservations since I used my pass cover number to book them? I have purchased the reservations for about €30 each for the Eurostar trips. Sorry if this seems like a silly questions but I love to worry about these things! lol
I have purchased the nightjet tickets through OBB nightjet website without my pass because I couldn’t figure out how to get a reservation. I think I checked afterwards and didn’t see any open seats for my particular day so its okay.
To get from Switzerland to Paris, I will look into alternate routes instead! Maybe Interlacken - Basel - Strasbourg - Paris? And then purchase reservations from Strasbourg - Paris through the booking website?
I really appreciate all the help! It’s difficult for me to understand the system without experiencing it but I’m sure once I’m there it will all make sense and I’ll be a pro! 
There is no direct connection between pass and reservations, they are linked by the name of the passenger in some cases, in others nothing at all.
The pass cover number requirement for some websites is only a method of stopping people without any pass from buying reservations and attempting to travel with no ticket.
You can deactivate pass now and re-activate it later without any worries.
