From Richard Paul Roe’s introduction:
Italian Shakespeare scholar Ernesto Grillo: “Frequently, [in Shakespeare’s plays] we find whole lines translated literally from Italian without the slightest alteration . . . .[O]ur poet,” he concludes, “most undoubtedly have had recourse to MSS In Italian.” Other scholars concur. Again Grillo: “ . . . [he] must have visited Milan, Verona Venice, Padua and Mantua.”
So from the start we have evidence that Shakespeare had access to Italian manuscripts, that he most likely read himself. It's far less likely that someone else who could read Italian would translate, verbally, into English an Italian work for someone else. If someone were to translate a work for an Englishman it would be more likely be a loose translation or a casual explanation, rather than "whole lines translated literally".
I will move through Roe's book one chapter at a time.
Highlights from Chapter 1 [pg 8]: (this first point now seems to me to have some doubt as a point of evidence but I'm including it because of its popularity).
1A. A troubled mind drave me to walk abroad,
Where, underneath the grove of sycamore
That westward rooteth from the city’s side,
So early walking did I see your son.
Was there a grove of sycamore trees at the western end of Verona?
Roe says yes, at the Porta Palio, one of Verona’s three western gates. He said that there were also remnant of sycamore groves elsewhere. Neither in Shakespeare’s main source of Brooke’s poem, or in any other version, is there mention of these sycamores. It’s original with Shakespeare.
Argument against: The sycamore trees are actually toward the South of the city, not the West.
Response: Roe shows a map indicating ‘Remnant Sycamores’ outside of the Northern end of the Western Wall as well as toward its Southern portion. The wall extends a long distance north to south and then turns East to produce what could be considered the ‘Southern’ wall. It would still primarily be considered the ‘Western’ wall even as the Southern end of it turns eastward. On the map that Roe provides the Porta Palio is on the Southwestern angle of the wall, with the wall angle well above 45 degrees, so that this section of the wall does look to be facing more West than South.
Other arguments against:
1) drawings from the 16th century don't show a grove of any sort next to the Western wall and that it would counterproductive to have such a grove since it could provide protective cover for invaders. 2) it appears that the trees currently in the area are not actually sycamores, but another species which have a resemblance to the sycamores. 3) Finally, an alternative reference to sycamores, and in a similar psychological context to that of Romeo's, existed in another literary work from which Shakespeare could have borrowed.
Responses could be: 1) the drawings may not be entirely accurate to all details such as a small grove of trees here or there. And it's speculative that such a grove couldn't exist because supposedly it would then interfere with the defense of the city.
2) I don't think it really matters whether or not they are truly sycamores. If they resemble sycamores and the author saw them and thought that's what they were, or if some traveler saw them and told the author about them, still there would be an accurate correspondence to explain. Unfortunately, Roe seems to only have been guided to them by his taxi driver, who may or may not have had accurate knowledge of the city’s arbours. On the other hand some taxi drivers are quite familiar with their cities. However, I remember being with one taxi driver who clearly did not know something quite well-known about his city, yet he was sure in his mind. So we can’t give much, if any weight, to the taxi drivers’ assertion. Now, Roe did try to verify they were indeed sycamores and from his knowledge they did seem to be so. So it seems they at least resembled sycamores and could be thought of such by the average visitor there. I think that’s sufficient.
3) this third argument, however, might be enough to doubt this particular piece of Italian knowledge. Romeo’s psychological context here with the sycamores resembles the other two times that Shakespeare spoke of sycamores, so that itself could account for him choosing that kind of tree. I think the strength of the argument would depend on if the trees that are sycamores or that resemble sycamores could be shown to have been there for several centuries. If so, then the coincidence of this connection would seem unlikely, especially given the many other accurate facts the author had of the cities there. But if such trees could also be found along other walls of Verona or If other Italian cities has sycamore trees or trees similar to them on their Western side then perhaps the observation was more commonly known and that would weaken Roe’s argument. So I think it could be a good argument for Roe but I’d just like to see some more investigation to it.