I'm looking at travelling from Milan to Brig in mid-June by train. At the moment, I can only see EC trains. I'm guessing timetables for slower regional trains will be released closer to June? Can you travel between Milan and Brig without reserving a seat - I will have a Eurail global pass.
Which site are you using to look?
Look at a date before June 10 and you will see regional trains with a connection in Domodossola. Takes longer because of a 41 or 51 minute connection time. Times should not change much after the June schedule adjustment, but you'll have to check when June rolls around.
Again, where are you looking? There are good and bad places to look, you may be looking in the wrong place.
The Italian Regional trains have not been released yet for the second half of the year (starting beginning June), but you can assume they will be as present. Swiss times have been released up to December.
I did a search on the Swiss Railways site ( https://www.sbb.ch/en ) for mid-May and got the following. This should be enough choice of trains.
Milano Centrale dep 08:23 1 h 53 min direct EC Brig arr 10:16
Milano Centrale dep 08:29 2 h 47 min 1 change R, IR Brig arr 11:16
Milano Centrale dep 09:29 3 h 4 min 1 change R, RE Brig arr 12:33
Milano Centrale dep 11:23 1 h 53 min direct EC Brig arr 13:16
Milano Centrale dep 12:23 1 h 53 min direct EC Brig arr 14:16
Milano Centrale dep 13:29 3 h 4 min 1 change R, RE Brig arr 16:33
Milano Centrale dep 15:29 2 h 47 min 1 change R, IR Brig arr 18:16
Milano Centrale dep 17:23 1 h 53 min direct EC Brig arr 19:16
Milano Centrale dep 18:23 1 h 53 min direct EC Brig arr 20:16
Milano Centrale dep 19:23 1 h 53 min direct EC Brig arr 21:16
The rule is in Italy, Regionalle trains DO NOT require a reservation, EC and High Speed trains do. In Switzerland, nothing requires a reservation. The only ones of the above trains that do not require reservations are the 08:29, 09:29, 13:29, 15:29. These are the slower options requiring a change of train.
I feel I must point out that a Rail Pass in Italy rarely pays, because Italian fares are cheap if bought in advance. And normal tickets always include a reservation if needed. For example, on the Italian Railways site ( http://www.trenitalia.com/tcom-en ) for 15th June, ALL the EC trains currently have tickets for €32.