The situation in early medieval Ireland was rather unique, as I explained in an answer to another question. The situation there was largely a legacy of the fact that the early monasteries had been founded under Irish Brehon Law.
The point made by M & H. Whittock about the attacks in Ireland seems reasonable, although the comment about Aldhelm, abbot of Malmesbury, seems slightly misleading in that context. In fact, as Barbara Yorke explains in Wessex in the Early Middle Ages:
"In the 680's Malmesbury received land grants from Baldred, a West Saxon sub-king, and from Cenfrith and Berhtwald, respectively kinsman and nephew of King Aethelred of Mercia ..."
These land grants meant that Malmesbury Abbey was built on land that was partly in the kingdom of Wessex, and partly in the kingdom of Mercia. This was a situation that could be potentially beneficial, but that also came with a degree of risk, in the event of war between the two kingdoms:
"Although Malmesbury's position in the border zone meant that it might benefit from the patronage of both sides, there were potential dangers as well. Aldhelm, abbot of Malmesbury, obtained a grant of privileges from Pope Sergius I which he got both Ine of Wessex and Aethelread of Mercia to ratify and further secured their agreement that Malmesbury should not suffer in wars between the kingdoms."
That is not quite the same thing as being "forced to negotiate a special arrangement with the kings of Mercia and Wessex to prevent them targeting his monastery".
The conversion of the Anglo-Saxon kingdoms in what would later become "England" was largely complete by the end of the 7th century. Which I accept that it is quite likely that there may have been attacks on Christian churches by raiders from neighbouring pagan kingdoms prior to that, I can't think of any targeted attacks on Abbeys or monasteries by neighbouring Christian kingdoms after that date. I spent a little time this evening reviewing the seventh, eighth and ninth centuries in my copy of the Anglo Saxon Chronicle, but couldn't find an example. I am less familiar with the Frankish chronicles, but I did a brief check online, and I believe the same is true there.
Viking raiders, however, certainly did seem to target monasteries. The first raid was on the monastery on Lindisfarne in 793:
"... on the sixth day before the ides of January in the same year, the harrowing inroads of heathen men made lamentable havoc in the church of God in Holy-island, by rapine and slaughter."
It is easy to see why Vikings would target monasteries. They were rich, isolated, and largely unguarded, or as we might say today, "easy pickings". The chroniclers who were recording these attacks were mostly monks - the very people being targeted by the Viking attacks. It is hardly surprising then that the Viking raiders were portrayed as "vicious".
The early Viking raiders in England were certainly "foreigners", travelling from Scandinavia to carry out their attacks. By the mid ninth century, larger armies were arriving with a view to conquest, rather than raiding. In 851, the Anglo-Saxon Chronicle tells us that:
"The heathens now for the first time remained over winter in the Isle of Thanet. The same year came three hundred and fifty ships into the mouth of the Thames;"
It is at this point that non-ecclesiastical sites - which were likely to be better defended - were being increasingly targeted. These were no longer raiders, but conquerors. In 867, one of these Norse armies captured York, which was then the second city in Anglo Saxon England:
"This year the army went from the East-Angles over the mouth of the Humber to the Northumbrians, as far as York. And there was much dissension in that nation among themselves; they had deposed their king Osbert, and had admitted Aella, who had no natural claim. Late in the year, however, they returned to their allegiance, and they were now fighting against the common enemy; having collected a vast force, with which they fought the army at York; and breaking open the town, some of them entered in. Then was there an immense slaughter of the Northumbrians, some within and some without; and both the kings were slain on the spot. The survivors made peace with the army."
These new Norse kingdoms in what would become the Danelaw were pagan. We know that there were raids from these kingdoms into the remaining Anglo-Saxon kingdoms. It is probably appropriate to refer to these raiders as "Vikings". They did attack monasteries, they were certainly "pagan", although perhaps not necessarily "foreign".
So, to answer your questions.
1. Why are Vikings singled out for raids in Britain and Ireland when
records may indicate most raids were by local rulers?
While this is certainly true for Ireland, there is no evidence to suggest that "most raids were by local rulers" elsewhere in the British Isles. For the rest of the British Isles, it seems that Vikings were singled out for raids because it was actually the Vikings who were carrying out those raids!
2. Has there been a similar analysis of the Anglo-Saxon chronicles
which will back up M & H Whittock's assertion that "We should not
expect anything different for Anglo-Saxon England", and has there
been any statistical analysis of the Frankish chronicles?
There doesn't seem to be any evidence in the Anglo-Saxon chronicles for the seventh, eighth and ninth centuries to support that assertion. I'm not aware of any statistical analysis of the Frankish Chronicles for this period, but I couldn't find any evidence to support the assertion there either. Furthermore, the quote from M & H Whittock's book in regard to Abbot Aldhelm seems to be rather misleading in that context, as discussed above. I'm not at all convinced that the assertion stands up to scrutiny.
3. The statistics refer to raids on ecclesiastical sites. Is there any
indication in the chronicles / annals as to whether
non-ecclesiastical sites were more or less likely to be attacked by
Vikings than by local rulers?
For the most part, outside of Ireland, "Viking" attacks on non-ecclesiastical sites are rarely recorded in the rest of the British Isles in ninth century chronicles or annals. Bear in mind that the term "Viking" actually meant something like "raider" or "pirate". Clearly, Viking raiders would tend not to attack well-defended, or fortified sites. However, these sites might very well be targeted by local rulers intent on expanding their territories.
Outside of monasteries, high-value items were likely to be held in well-defended, fortified sites, so Viking raids targeted monasteries for their wealth, and smaller undefended settlements for slaves. We have nothing in the chronicles to suggest that local rulers attacked monasteries, or that they raided neighbouring Christian kingdoms to capture slaves.
When the Norse armies begin to arrive with a view to conquest, the term "Viking" is no longer really appropriate. It is at this point that we do begin to see attacks on non-ecclesiastical sites appearing in the chronicles. However, this is a very different situation from that in Ireland, where there are a number of Norse coastal settlements, but where there was never a concerted effort to conquer the island.
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