While there was certainly a collapse of trade with the east(In 600 the graves of the Merovingian kings are decorated with Byzantine jewelry; this disappears later)
Mainly because Merovingian kings used local production. I don't think that the development of proper work in Gaul, avoiding to have byzantine decorations, could be defined as "collapse".
Furthermore, the disapperence of oriental (mostly byzantine) goods was more localized in the 700's and was not even complete (Charlemagne had a royal byzantine tapestry)
And a general stagnation after Clovis in the Merovingian empire one can't really call it a generic blob. If you must, separate it into two with the reign of Charlemagne in the middle. Truly he breathed life back into western Europe and reopened much of the trade.
General stagnation? Are you sure? Admitted the agricultural production stagned because of colder temperatures (since the 300's, really perceptibles during the 400-600), but the (re?)-appearance of a gift-based microeconomy used others flux than trade without representing a lesser alternative.
In fact romans roads have probably been abandoned for trade because of new techniques of transports as the trade still existing, but in slower flux, used more capacitying of carring.
Regarding Charlemagne, he persued only his fathers and great's father policy of general raid, in order to perfuse the gift economy.
After him, we can perhaps mark it into an era of terror, anarchy, and constant raiding, with the Saracens, Magyars, and Vikings attacking the cities of western Europe.
Since Charles Martel to the death of Charlemagne, we can count : 4/5 raids against Aquitains, 4 against Lombards, 2/3 against Frisians (sort of Venetians of North Sea at this era), 3 raids in Spain (including raiding allies), 3 campaigns in Saxony, plunder of byzantine possessions, raid against the Avar Ring.
Where the big difference with the post-carolingian raids regarding the constance and the violence?
For the trade and agricultural revolution, it began really only after the death of Charlemagne, as the nobles had to find other way to have luxury goods than just raiding neighbors : by using another fiscality than the outbassed roman one, by making their domains productive and by enforcing trading in order to get "exotic" goods.
If these times weren't ones of economic change and growth, i doubt that Vikings and Maygars would have raided that much.
For cities stuff, no. Vikings attacked countryside first, not far well defended cities. They did so when armies retranched itself behind the walls, ready to attack them soon they began to plunder.
For Sarassines, it was "only" a slavery piracy, very limited regarding targets and inflicted damages.
Still, monasteries held great knowledge within their confines and the brilliance of Byzantium and Muslim Spain(both in the second period) heavily influenced Europe.
Monasteries held critically their power and influence from the markets and peasant fleeing feudal violence that ioncreased the productivity of domain. Little chance to them to became centers of knowledge if they didn't managed to keep themselves relativly independents from neighbors.
May i ask why are you quoting Byzantium and Al-Andalus that are maybe the less influences in the IX? Even in Catalonia, at the very own contact of Spain, technological and cultural changes would have to wait the XI to really occurs.
FOr Byzantium, the fact that clerks keep greek texts and commentaries are not meaning that thek kept contact with Constantinople. At the contrary, we began to see wrong ideas about orthodoxy church, about ERE at this date, when Clergy and Papacy were definitly seeing their hope in the Franks.
On the topic of Ireland, I had always felt that it's 'golden age of Monasticism' had begun to decline soon after the bid to convert the Ango-saxons to Irish Catholicism had failed, but perhaps I am wrong.
They didn't even managed to convert them, mainly because of ties between monasticism and nobility in Ireland, and in Britons regions.