Don't worry about the DB trains too much. Also, don't use the Interrail website for reservations if you can. It's not so reliable and more expensive.

Here's an overview of where's best to book:

https://www.seat61.com/interrail-and-eurail-reservations.htm

I suppose you're looking at this journey:

Book Eurostar on www.raileurope.com, so you don't have extra booking fees. €30 in 2nd, €38 in 1st class.

Book Brussels-Frankfurt on tickets.oebb.at: €3 per seat.

Book Frankfurt-Salzburg on bahn.com in 2nd class (€4.90 per journey, use "Book seat only”), or in 1st class on tickets.oebb.at (free of charge, book each train separately, add the Interrail/Eurail discount and the select "One-way tickets”, NOT "Seat reservation only”). These 2 trains have optional reservations.

Just to let you know so you don't book for the wrong date : it's Friday 19th July and Saturday 20th.

Those connections are pretty tight but there are plenty of later trains in case. Knowing German trains it's also possible that the connecting train is late as well... :)

For the Nightjet on 9 August, it's too early. Booking is a bit easier on www.nightjet.com, where there's only one method (add a discount).

The night trains to Brussels and Paris however can often only be booked about 3 months in advance; other Nightjets up to 6 months.

Thank you so very much for all of that great information. Oh yes, realised I put wrong date, yes Saturday 20th 

So the journey I looked at was very similar, but gets into Salzburg at 22.02, so I guess slightly later German trains, which would make sense, being you said other was quite tight.

So when I book the German trains last leg of journey, what happens if I book the ones you suggested with tight schedules, and miss them, so have to wait for the next ones? Am I still ok with interail pass (no extra payment?)

Assuming I just miss out on any reservations if I have reserved seats. Is there really any point in reserving seats on this part of journey?

Just to clarify on our return the Vienna to London train I am going for Vienna to Brussels sleeper train (seemed cheaper than Paris one).

Assuming I need to be on to this as soon as it becomes available as I need to book a 3 sleeper compartment, and I am guessing that there aren't many of them.

Before Vienna (where we plan to stay a few nights) we will coming back from Ljubljana which seems pretty straightforward.

One more question, if you don’t mind, 

After a stay in Budapest during 3 weeks we will be getting 9am Budapest Deli to Ljubljana 4.42

I am looking at this train as seems most straightforward, and going from there same day to Bled (6.03 or 6.53 train). Someone told me it's really slow, and have food and water, but that's part of the fun and experience!

Does that train seem a good option to you?

Many thanks for your support. My husband is happy to stay at home looking after the sheep, dog, cats and poultry, especially after all this worry with the planning 

If you miss a train in Germany, generally just get the next one. Reservations are optional. Get the DB Navigator app to see delays and cancellations realtime.

Also assuming trains Salzburg to Budapest, Budapest to Ljubljana, and Ljubljana to Vienna are all fairly frequent, but if I am hoping for certain times, mostly earlier mornings, then I should book/reserve seats?

Thank you 

During summer I'd always book seats for a family. No need to do that months in advance though. 3€ per person per train through ÖBB.