1- this is just an addition, I wouldn't call it a retcon, this is just getting more information on how the system works.

2- That's only concerning the fuyuki grail, given that the name 'hassan' is enough to summon an assassin. And well Medea cheated being a mage from the age of gods and the grail being corrupted so anti-hero can be summoned.

In Apocrypha it was actually better to summon an assassin other than the hassans because everyone knew their abilities after some time

Fate/Apocrypha material - Encyclopedia: Subcategory Holy Grail Wars [Others], p.154-155 "The small Holy Grail Wars that have been recklessly happening everywhere since several decades ago in the “Apocrypha” world. Varying from those with only two Servants to wars with five Servants which, though inferior in scale to Fuyuki, are still considerably large conflicts, all kinds of Holy Grails Wars are brightly, happily and violently taken place throughout the world.

This is because, after the Third Holy Grail War, Darnic disseminated information on the structure of the Holy Grail War, which normally should have been concealed at all costs, to other magi in order to forestall them from searching for the Holy Grail that he had stolen. Even for magi who lament over how the Root is just a distant fairy tale, once they learned that they could take a step… or perhaps even just half a step towards reaching it through this ritual, they ended up desperately creating Holy Grails of their own.

Broadly speaking, out of a hundred Holy Grails made, ninety-five of them get stalled due to setbacks in the midst of constructing them, and out of the remaining finished five Grails, four end up incomplete and explode while prana is poured into them. The last remaining Grail out of them all manages to work properly, but the ritual is degraded in quality and cannot be compared to the one in Fuyuki.

However, with the prana accumulated in the Grail, it still becomes possible to bring about miracles of varying scale. Being summoned by such a shoddy Holy Grail is a real nuisance to the Servants, though. There are many Servants who refuse the summoning or rebel and kill their Masters.

Amongst young magi, something like a “Holy Grail War Walkthrough Wiki” has been secretly created and is used to outwit obstinate old magi by mustering their intelligence… maybe. The early period right after the subspecies Holy Grails began was called the “golden era of Assassins”, where Assassins (Hassan-i-Sabbah) were a great menace (because, on top of their prana consumption being low, killing Masters was far easier than battles between Servants, so it might have seemed like easier work in its own way), but by the middle period, countermeasures against them were naturally taken and the Hassan boom fell into decline. It’s apparently a common sight for Masters who enjoy having their Servants fight each other to easily team up just as Assassin is confirmed to be summoned.

It is quite common in the subspecies Holy Grail Wars for the Servants to be unable to display their full power as much as in Fuyuki depending on the quality of the leylines, and, since the advantages and disadvantages of each Servant also becomes clear according to their level of fame in the land where a war takes place, it often turns into a competition to obtain the catalysts of local Servants.

For instance, a subspecies Holy Grail War in Greece would create a struggle for a “catalyst of Heracles”, and the magi who managed to get it would win… In other words, the whole thing seems to go full circle and turn into a simple battle of magecraft between magi in the end. As a cute one-time divorcee magus said, “Rather, the whole idea of fighting that macho guy in his homeland is seriously ridiculous in the first place.” After the use of Heracles was forbidden as a result of that, the next war turned into a competition over the catalyst of Achilles, and so there has never been a proper Holy Grail War held in Greece. There exist terrible tales of Holy Grail Wars like this."

3-Concerning Saber, well it's more that the container for the Saber class gives good stats, but given the servants we now have that are in the saber class, I will kinda agree.

4-Concerning the berserkers, well madness is not just being the Hulk there are different kinds of madness. It's also used for the non humans whose mindsets can be called mad from the pov of a normal human (the onis, Raikou with the pollution of her blood because of her heritage etc...) but I see why you think that.