Light Novel vs Anime

@Hiro_Ayuzawa I don't think it was ever said that you can't, however, it's probably nigh impossible to learn even 2 different ones without some kind of seriously underhanded trick about your own body. I doubt anyone body's would resist using several of them for long.

To be clear, spellblades are way more than a regular powerful attack. They bend the laws of reality to achieve that cheat-like one sure hit KO that is only made possible by having the magic field surrounding the user being exactly in tune with that reality bending. So, I don't know if it's clear or not by now, but all mages basically alter reality around them passively. In which manner is dependant of genes, mentality, and so on. Every mage is different. And a spellblade is the ultimate product of those uniqueness. So to use the spellblade of someone else, you would have to literally be that someone else while using it, in mind, in magic field, in body. This is why Oliver's spellblade is destroying his own mind and body whenevr he uses it. It's a borrowed spellblade, not his own. And Oliver has a double big advantage compared to anyone else, one thing he is close of his mother in genes obviously, and the other being something that is yet a spoiler. So ... I don't see it really being possible unless you're willing to destroy yourself mentally and physically on the long run. Borrowing one and using it with parsimony, taking enough time for healing your body and soul in between each use, as Oliver is showing, it's possible provided you've got very, very peculiar circumstances. Borrowing more than one? I can't see that happening. Especially since you would also have to find who has the others and find a way to steal it for them fully.

I will try to answer both of your questions.

1) The adaptation is better than most LN adaptation as post #3 mentioned above. There are some skip content such as magic comedy, dialogues and gore.
2) Spellblades is a ‘cheat’ technique that only 7 people wield. The main reason Oliver struggles with his Spellblade is mentioned above in post #5, the Spellblades do not belong to him, he did not inherit/learn from her mother. All these information will be explained in Volume 5 when he goes after his second target.

The author has a habit of explaining the reasoning much later, similar to Tenkyou No Alderamin. So be patient with it. :)

Spellblade is a ‘cheat’ instant-kill technique that only a few people possess. Every Spellblade is different.
Most of Oliver’s powers will be explained during the fight with his second target.
Most of Spellblades are not explained in the LN yet. As of Volume 8, only 3 types of Spellblades have been used.
How Spellblades are created have not been explained yet.

Oliver will go after his second target at the second half of Volume 5. The anime will likely end at Volume 3.