It doesn't work, just like transferring the copyright to a young person to make it last longer doesn't work.
In places where the length of copyright depends on the death of someone, it always depends on the death of the author. You can transfer copyright, but you can't change who is the author. If I write a book, and some copyright law says the copyright ...
Under Swedish copyright law, a work such as a movie is protected for 70 years after the death of the "creator". It is unclear who the copyright holder is, but it has not been 70 years since the film was made. Unless it was explicitly "released into the public domain", it is still protected, so you can get sued.
No. The dead are generally not considered legal persons. They have no rights, and cannot own property, make contracts, sue, and so on. Giving them copyright would make as much sense (legally) as giving your rights to Niagara Falls.
(Even if you could give copyright to a dead poet, it wouldn't help. As gnasher points out, copyright usually runs for 70 years ...
I would recommend writing the author and requesting permission. There's likely not much anyone would do if you refused, but you could certainly lose access to the site, and it sounds like the site is helpful for you.
From their TOU:
Restrictions on Use of Content
If you purchase Content from the Site (including receipt of free
Content), you agree ...