Thoughts: I am certain that "Northmanni" has to be a plural subject
for the verb "intraverunt" in this sentence, but I am unsure about the
verb "pervenerunt". It seems clear that this verb is still referring
to "Northmanni", but I am a bit unsure whether the subject for this
verb is inexplicit due to the "et" creating a new clause.
I am not quite sure what you mean by "inexplicit"; however, either the subject of pervēnērent is present explicitly in the sentence, or it has been dropped as understood or implicit.
In the context, Thurones seems to be a good candidate for the subject. I could not find this word spelled this way, but did find Turonēs as an ethnonym for a Gaulish people living in the region near the modern city of Tours during classical times.
I believe it was common in Latin and in many Germanic languages to use an ethnonym metonymically for the principle city or region of a people (e.g., "Paris" from Parīsiī, "Reims" from Rēmī, "Cahors" from Cadurci, English "Wales" from Walas (Welsh people), "Sweden" perhaps meaning among the Swedes, and German Polen (Poland) perhaps originally meaning among the Poles.) With this understanding "Thurones" could be a spelling of "Turones" and refer to either the people, their region, or one of their cities, Tours.
In the context, the previous clause talks about the Northmen entering the Loire River in their boats, which is where Tours and the Thurones once were. Given the context and the late date, we can assume from the first that "Tours" is being described, since I am not sure that the Turones would still existed at the time of the events described. The next word, usque, further disambiguates the meanings, giving us "and (they) reached as far as Tours, ravaging everything," referring to the actions of the Northmen/Vikings/Norwegians.