The Vendel era helmets were pretty similar to the Anglo-Saxon helms in terms of overall design. But as far as I know none of these helmet types remained in use during the Viking era. The only surviving example of a "Viking helmet" is the Gjermundby helm. Although the conical "Norman helmet" is very common in art, and was probably the most common type during the Viking era.
Regarding sound changes in the Anglo-Frisian languages: read this. Note that the Saxon dialects do not belong to this group. The Saxons of the 5th century probably spoke a dialect that was the forerunner of the 8th century Old Saxon. However, the ones that migrated into England gradually adopted a new dialect, as a result of Anglo-Frisian influence. The "original invaders" were probably a mixed bunch who spoke a variety of dialects, rather than a unified people speaking one language/dialect.