I'm going to take your questions in order, but I'll also address what I think are some misconceptions in your question. I might not be able to cover all aspects of everything you've asked since it covers such a wide area and touches on so many aspects.

Let's starts off with the idea that conversion kind of stopped in the 5th century and only picked back up later. This isn't accurate. Missionaries and bishops were actively working in pagan lands throughout this entire time frame. The 600's saw the conversion of the Anglo-Saxon kingdoms from paganism. And the 700's saw extensive missionizing in German lands, Saxony comes immediately to mind (aided by Charlemagne's conquests). Missionary movements to the Slavic peoples also started around this time.

However you're right that quite a few pagan societies adopted this new religion at this time, rather suddenly in some cases. So what were the reasons for this? Christianity offered numerous practical benefits that surely must have been on the mind of those converting. I'm hesitant to say that these practical concerns were the only factor, but they clearly played a role. This is perhaps one reason why Kings were so eager to adopt Christianity. It greatly aided centralization efforts.

One of the most important benefits of Christianity was the adminstrative expertise that it brought. These societies that converted lacked an organized adminstrative structure, and Christianity with it's dioceses, bishops, and perhaps above all literacy, filled this niche quote expertly.

Conversion was also often a pre-condition for peace between pagans and Christians. Alfred the Great only accepted peace with Guthrum, leader of the Great Heathen Army, after he underwent Baptism. Adam of Bremen reports that the Danes converted to Christianity after being defeated as well. Vladimir converted as a part of an alliance with the Byzantine, he also received an imperial marriage as a part of this.

There very tangible benefits to conversion for pagan rulers, that they could not get by remaining in their old traditions.

Now what did this process look like on the ground? It's all well and good for the king and his fellows to convert, but how did the populace experience this? There are any number of hagiographies and letters from saints that describe the trials and tribulations of life as a missionary. Miracle working of course features prominently, but so does prostelyzing. In addition to this, there are famous examples of Church authroties, notably Gregory the Great who favored gradual syncretism and overlooking minor doctrinal disputes. In both Bede's history of England and Boniface's correspondence this comes through quite clearly. Some pagan practices, and structres and sacred spaces, could be coopted into their new religion, but the more onerous practices, infanticide, animal (not to mention human) sacrifice, and so on were to be repressed.

Now opposition. Hagiographies and church accounts are full of repentent pagans who work against their former religion. Bede mentions one incident of a priest destroying his own idols after converting, and stories of Widukind in Saxony also mention his own conversion. Unfortunately we cannot rely on many of these accounts since they are inherently biased and quite possibly do not reflect reality. However in general there was little viable opposition in the long run. In Anglo-Saxon kingdoms there were often generational divides when it came to conversion according to Bede. The Kings who converted often left pagan sons who then returned to their pagan ways, but eventually they lost out. The Sagas, not to be relied upon either, mention similar processes in Scandinavia especially under rulers such as Hakon of Norway and St. Olaf (both figures are somewhat fuzzy in our understanding of them and are also tainted by later political concerns).

I know I haven't been able to answer all of your questions, and I've skewed my response a little Northern iver Central or Eastern Europe. However if you've got questions, want more sources, or some secondary literature, I'd be happy to provide you with the ones that have formed the basis of my answer.