Well I can't debate on the fact that Ryukishi really gave a strong message with ep5: "this story is solvable", "reasoning is possible". And he also added a not so subtle message: "refrain from devising crazy theories". He went as far as to say "(This is not Higurashi), there isn't any "hinamizawa syndrome" there isn't any "yamainu special corp".
In fact you could say that in Higurashi Ryukishi broke several Knox rules, and because of that people thought it was quite possible that Ryukishi would do the same for Umineko, even I thought that.
It is funny how ep5 focuses on something that was foretold on the very introduction of Umineko
http://witch-hunt.com/hist.html
Is guesswork possible or impossible?
However what I'm saying is not that this game is completely misleading. This game was created by Beatrice, and Beatrice's purpose is not to make Battler believe in witches, it was designed to make Battler get to the truth. In other words there are rules that make it so the gameboard must be solvable in the same way the first story was solvable. When Lambda took over, she couldn't broke the rules set by Beatrice, but she could manipulate some elements in a way the solution was almost impossible to be found. Kinda like the same way Bern warped the red truth to get to a completely false statement.
There is also what Dlanor said: "You can't make a piece do something that it can't do". Frankly I trust Dlanor more than Bernkastel. So when Bern said that piece Battler solved the riddle
only because she was playing him, it's not true. According to Dlanor Battler does have the potential to solve the riddle.
With such premises it isn't really important that the gameboard is in the real world or in a magic world. As long as it helps Battler (and us) to get to the truth.
Anyway clearly if you think the gameboards are in the real world, then you need to accept a certain degree of "magic", because traveling between kakera and or resetting time is not possible. But if you think the gameboards are in a fantasy/metaphysical world, then it isn't magic at all.
Beside, everyone is assuming that the "time loop" in Umineko is the same as in Higurashi. But is it really so? Traveling through kakera is Bern's ability, was it ever said that Beatrice has that power? Beatrice is witch of endless, but at this point I'm almost certain that Beatrice's power is the power of imagination. In her world, what is imagined get materialized. However if the means to imagine are destroyed in the real world, they get destroyed even in her world. Maria couldn't imagine Sakutaro was still alive, because of the trauma she experienced. With the imagination you can get everything you want, you can get all the friends you want, you can have happyness. But imagination can also be used to "kill" someone you hate, and then kill him again again and again until you are satisfied. When certain conditions are recreated, the imagination can become delusion, it crosses the boundary of a single person and spreads to other people. That is what happened with Kinzo, and also the legend of the witch.
Virigilia called magic a "white lie", Dlanor said this "magic" isn't necessarily bad, but it can be used for evil purposes.
Now if this is really Beatrice's power, then the time reset doesn't really happen, it's just Beatrice's imagination, there's no way for her to reset time or travel kakera.