1) I'd call the storyline of TOME convoluted at best. At worst, as bad as the art that appears between menus and loading. To divvy it into something, I would say the story of Tome consists of three parts: The distant past (Necromancers, Spellblaze, etc), Factions, and Now.
The past of the world of TOME is as I vaguely understand is one where magic ran free, and Necromancers were a major faction at the time. Then the Spellblaze happened which piledrove the world into a dark age, which it only recently recovered from.
Now, there's basically rumbings of a "bad thing", so you depart from wherever and according factionally, go to deal with said bad.
The factions as such can be divided according to the classes and starting locations, but basically:
Magic, Antimagic, Time Mages, Undead, and the Yeeks.
Basically the undead want everything dead, the Yeeks are weird ewoks, and the Magic/AntiMagic factions butt heads regularly. And Time Mages exist outside of time and typically work to make sure the continuity doesn't get unraveled or something.
Oh, and there's an extra story campaign in the DLC involving the aftermath but dismissive spluttering.
2) Is Tome4 beginner friendly? Heck no.
Tome4 has these fantastic multifaceted problems. It has too many stats and yet has no idea how to present them in a human readable way, the majority of players are playing on the massively overtuned Insane difficulty, and a good number of classes suck to the point of complete non-usability.
Also, several of the better classes and races have unlocking criteria so absurdly esoteric as to completely ruin the point of attempting them.
3) Grinding in TOME4 is nigh impossible. In the actual game campaign, dungeons will get cleared out and remain that way; plus they'll spawn enemies attuned to your level upon entry. And in the other game modes, there's no way to grind either; except maybe if you do something dangerous and breach a vault.
4) The overworld is preset except for the location of optional dungeons which will throw themselves wherever they feel like it. The actual dungeons are procedurally cobbled.
5) The multiplayer component of TE4 is completely unknown to me.
6) Replayability. Depends. Do you like infinite dungeons, testing out builds in the arena, or tying different classes? Then yes. If you're only going though the story, then absolutely not. Thankfully
, there are mods which can fix two of the problems
I've pointed out.