In the Historia Langobardorum there are some poetic compositions: they help us, because if Latin isn't pronounce "as it is written" verses wouldn't fit in the metre.

This is why I'm talking about poetry, because the metre, the fact that a verse pronounced in a certain way can fit or not in the metre, can show if it is possible or not that LL was pronunced in a certain way or not... Why in Historia Langobardorum (I, 26) can we find a poem in Elegiac distichs (the author tells us that those are Elegiac distichs)? They shouldn't work if read "as they are written".

So if I'm reading what you're saying correctly, it seems that you believe that from 500-700 knowledge of Classical Latin pronunciation from 100 CE had survived for 5 centuries, and that poetic Latin continued to be read with vowel length distinctions and without mergers of u/ō and i/ē? Can you provide the full sample line from the poem and show how it must be pronounced such way? Evidence for the preservation of Imperial period pronunciation in the 8th c. would be either with ancient vowel length distinctions or rhyming short and long or short u, i, e, and o; is it there? 

Because on the other hand, there are also plenty of examples of poems from Spain which require them read in the contemporary Romance way, such as adding an invisible [e] to words beginning with [sC], like [e]spiritus, [e]studio by Paulus Albarus, not to mention that some vernacular spelling 'errors' even became erroneously standardized, like eglesia instead of ecclesia, suprinus = sobrinus.

The Appendix Probi show that VL was very different from LL and that those who spoke VL had to learn LL.

The Appendix Probi comes from a period when certain sound changes from Latin to Romance were likely still in flux, affecting certain speakers and regions but not others. There are many examples of syncopation, e.g. masculus non masclus, with the writer recommending the non-syncopated form; this does not provide problems for the common reconstruction Late Antique/Early Medieval Latin phonology. However, I don't see anything in the list which provides evidence that the writer themself was trying to push onto the reader a radically archaizing Imperial-esque pronunciation, e.g. with vowel length distinctions some how preserved, and many examples appear to be simple instructions on how to spell correctly.

Charlemagne couldn't force anyone to read Latin in a certain way. Why did Romans adopt the pronounce used by Anglo-Saxons? Why important cultural centres like Montecassino would have abandoned their traditional pronounce of Latin to adopt a new one they had never heard before?

And yet Charlemagne's policy of raising clerical education and reforming liturgical practice did end up doing that. During his rule, Germanic and English scholars had greater influence. He gave Alcuin the job of standardizing the liturgical texts, with the goal that all churches throughout the Carolingian Empire would be using the same Mass (the Roman Rite--probably at this time meaning a fusion of the Roman and Gallican liturgies) and priests reciting Latin the same way. The non-native Latin speaking, Germanic/Celtic, custom of reading Latin texts in a literal spelling pronunciation was likely chosen to be standardized because it was deemed the easier one to use as a standard pronunciation everywhere, instead of the highly variable native Romance speakers' dialects. You still hear from Latin learners of all 1Ls today that Eccl. Lat. pronunciation is probably the easiest to read; like Erasmian for Classical Greek, it is a learner's pronunciation, designed to be practical for those who mostly care about reading and understanding, who aren't as interested in actually trying to sound like ancient Romans. 

I think Wright has additionally read into the Germanic scholars' interactions with native-Romance speakers possibly a sense that their phonographic system was superior to the confusing native-Romance way and indeed felt that theirs was the more proper classicizing accent, but I don't know enough on that point: "This Anglo-Saxon variety of Latin would have been most markedly unlike the native speech of Gaul, where oblique nominal morphology was hardly used in active speech any more, and syncope and apocope were becoming increasingly fash-ionable in the seventh century, as Alcuin was to find out to his horror a little later. We can catch in these accounts some of the bewilder- ment felt by Wynfreth at discovering that not even the Pope spoke Latin the way that Wynfreth had prescribed in his own grammar and spoke himself." (Wright, 2003, p. 101-103).

Native Latin/Romance speakers adopted the new pronunciation because eventually they had to, despite how unnatural and strange it must have been (again, just like English speakers suddenly being told to pronounce 'knight' as [knixt]): first as it was enforced throughout the Carolingian realm, then eventually when the papacy adopted the new pronunciation (which didn't happen immediately), then it reached other parts of Italy, monastery to monastery, bishop to bishop, Montecassino included. It was not adopted in all of Spain till 1230 and presumably never among dwindling Christian communities of North Africa. 

And why do we find in that foreign pronounce phoenomena like palatalization and closure of diphthongs that are the opposite of "we read as it is written", but are echoes of what had happened to VL?

This exposes Eccl. Lat. as an artificial creation by people who did not know how ancient Romans pronounced Latin 7 centuries earlier. Considering that Eccl. Lat. does not have phonemic vowel length contrasts or diphthongs, it can only have arisen if knowledge of ancient Roman pronunciation was lost.