That is really interesting HB. You really seem to be having a great time exploring, in ways I have never thought of. You should post more with photos if possible because I am sure that a lot of members would like to hear more.
If you download Interrail site NGB, it looks quite daunting at first, but you can work out cost's etc, but when you download the App for Interrail it becomes quite easy. The only thing with Eurostar, you have to check that room is available for an interrail pass.
Check in with Eurostar was quite straight forward, i took the Brompton with me so i didn't have to book a cycle space [travelling with unfolded cycles on Eurostar is not straight forward] my Brompton carried my Luggage for me where ever i went, because i want to go for longer and this was a taster for me, i limited myself to 40 euro's a day for food, i could have gone more expensive, but as a traveller on the move i wanted to see if i could do it, and it was easy to do and i ate quite well.