I'm not sure where to put this... but here goes.

I've been reading some (actually a lot) Harlequin manga though I haven't read any of the novels they are based on. Now, I've got a question for someone who has. Does all the problems get solved in the last few pages in the originals as well? Are there NO tragic/bittersweet endings in these? I mean, no matter what troubles they encounter, miscarriages, murders, illness, kidnappings, amnesia, dying (yeah, I'm exaggerating 🙄 ) is always solved without anyone really getting hurt...

If there are any, either a manga adaptation or an original novel, could some kind soul point me towards it?

~4400 manga completed
~1700 ongoing...

Yes. Harlequin = happy ending. I doubt that sad/tragic ending even exist in their dictionary.
Some character might have unhappy ending, but it only happen when:

1. The character is the main character's love rival/ antagonist
2. The character is supporting character, and the novel is a series. In this case, the story will continue in the other series with the he/she as main character and end up in happy ending. Correct that: end with marriage or lovers.

I've read some harlequin novels with the main character has disability, but it always happen at the very start of the story, and is a plot device for the author. Like always, it ends with happily ever after.

Yep. I read a bunch of Harlequins in Jr. High, and they are always, ALWAYS happy endings for the main characters...which wouldn't bother me if the happiness was a bit more realistic, but often (always??) it's pretty last minute & contrived. Haven't read a Harlequin in years (and don't expect to read one any time soon) but I'd be extremely surprised if they've changed much.

"[English] not only borrows words from other languages; it has on occasion chased other languages down dark alley-ways, clubbed them unconscious and rifled their pockets for new vocabulary."
-James Nicoll, can.general, March 21, 1992