The advice is that you don't activate the pass until just before boarding the first train of your travel. Please read more about that in my next reply.
Adding trains as journeys to your Trip in the pass has no connection to the train companies. It is only something between you and Interrail/Eurail.
You should start by making any reservations that you need and you don't need an active pass for this.
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 you choose the start day of the validity of the pass. Once the validity has started it can't be changed even if you haven't travelled. The advice is therefore to wait with activating the pass and starting the validity until the first day of your travel as you only can deactivate the pass no later than 23.59 CET on the day before the validity starts. If your travel plans change in the last moment you can't deactivate the pass and change the validity.
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.
If the grey ledger can turn yellow, you successfully connected your pass to your trip. But only add journeys to your pass, just before boarding the train. It's always possible you'lld want a train earlier, or later.
Best deactivate everything, in order not to risk the loss of a travel day. Only activate it just before travelling, you never know ;)
As you said yourself, reservations are totally separate of the app. Adding journeys to your pass won't do a thing, it doesn't reserve anything. It just registers your train to your pass, nothing more.
Only by reserving a seat, you secure a seat on a specific train.
Trains do not sell out that quickly: some exceptions though. If you plan taking the Eurostar or Night Trains, book it as soon as possible. For most other trains with mandatory reservations, there are alternatives and they don't sell out that quickly, but Eurostar is difficult to avoid, sells out quickly too. 
Night trains are very popular at the moment, and can sell out motnhs in advance on some stretches.
What's your broad travel outline?
And on busy summer weekends, the so called "switch weekends", when half of Europe comes back from holiday, and the other half leaves on holiday, it's quite pleasant to have a seat reservation on trains where it's possible. During the week it is usually better.
Adding a journey to your pass is the equivalent of writing a train in the trip diary of a paper pass. It's just registering your journey to make your pass valid for that train, nothing more.
