Acheron question

I read this as a stand-alone. It's the only book in the entire series I read, and I'm fine with just that.
I thought everything that took place in the "past" was amazing. When it switched to over to "present time," not too impressed.

You can, but there's a ton of stuff you'll miss. It'll also spoil the building mystery of Acheron through the first 14 books.

Fantasy Lover is not the official first book, and you're totally fine skipping it. I did, then went back and read it after I'd plowed through half a dozen or so and wanted to get the scoop on Julian met Grace.

I would recommend starting from the very beginning. I got the box set on sale only knowing I was saving a ton. Best purchase ever! The Dark Hunter verse is huge so if you really have to cut corners I would recommend the actual Dark Hunter books. Acheron is the eighth but was the fourteenth book I read in the set. And I though Fantasy Lover was a good lead in even though it's technically not a DH book. But Acheron is a dynamic character that you like right away and then slowly unfolds over each book until his own. So I'd definitely recommend trying to commit to at least the core series.

I read Acheron 1st and it was fine for me. Afterwards I read book 1 and 2 and I don't know why, I wan't feeling it. Acheron is definitely in a class of it's own compared to the other Dark Hunter series I read. It is so sad and heartbreaking.
Maybe if I started the series in order I would be appreciating it better but ultimately, Acheron is a must read. Reading it alone won't interfere with your appreciation for the book.