The train from Brussels to London will use your inbound travel day for France. You will then not be able to use the Eurostar to return to France or any other trains in France with Interrail.

You need to decide which 2 days are best used in France and then either pay for seperate tickets in/out of France or use alternative routes or ways of travelling for any other trips that would involve trains that travel inside France.

Eurostar requires a valid pass in al countries you pass through, other wise you'll have to buy a normal Eurostar ticket. For the journey to Zurich, you can buy a ticket to the border (Basel SBB in this case) and use you pass for the rest.

You did not tell clearly where you live in that hexagone served by that state-run SNCF.

You can go from the Swiss to BE via Germany-LUx and thus avoid to travel in that country where you speak your own language-

For this €*: depending on when you actually go, it might not even cost much more as a normal ticket-with pass you have to pay at least 30€ extra too. Normal tickets go from 39-as it happens just 2 days ago I used €* myself from LON-on the site of €* itself is also a ´low fares finder tool´ (it probably also in fr), then you can check what days are cheap-never weekends.