Interesting idea, and I decided I would take a look. Here are the walking distances in miles per Google, with straight-line distance in parenthesis:
- Walpole: 4.4 (3.5)
- Sharon: 6.5 (4.5)
- Mansfield: 6.7 (4.8)
Walpole seems the most promising in terms of distance, but all three suffer from being very pedestrian unfriendly routes, with lots of walking on high speed streets or weedy shoulders. I can't speak for Europeans, but as an American who enjoys active transportation, these are not walks I would want to take. As for Ubering from the stations, maybe. But I suspect there will be huge uppricing for Uber activity around the stadium on match days. That charge plus the regular MBTA zone charge might work out to a similar cost as just buying the event specific ticket. I'm not even sure there will be any non-event trains at those stations at times that would work, given the schedule adjustments.
Fundamentally, World Cup attendance is going to be super expensive. People who are able and willing to pay that cost will probably also be willing and able to pay $75 to get to the stadium.