Since yesterday, there's a problem with the Interrail reservation service, so please wait for a fix while they fix it.
If you want to check normal fares, you can check them here:
- Basel - Firenze: SBB, Trenitalia
- Amsterdam - Brussels: NS
- Brussels - Mulhouse: SNCF or SNCB (via France), DB (via Germany, enter via Cologne)
- Basel - Salzburg, Salzburg - Genève: SBB, ÖBB
- Genève - Basel: SBB
- Most of the above: Trainline
As to reservations for Interrail, they're not needed for all of these journeys. For day trains:
- Basel - Firenze: needed for the long-distance train from/to/in Italy.
- Amsterdam - Brussels: not possible for the hourly intercity, mandatory and expensive for Thalys.
- Brussels - Mulhouse: needed for long-distance trains via France; not needed via Germany.
- Basel - Salzburg, Salzburg - Genève, Genève - Basel: not needed
An overview of reservation prices is here.
Note that currently=this month, there is a 10% discount off all global passes-the total price can be found on the site. Depends on your age. Note that most fares on railwaysites are fixed for a date/train and will rise the longer you wait with booking-samesame as SNCF does with TGV prices.
Update: the issue with the reservation tool has been resolved so you should be able to use it again. Cheers,