Harlequin's unwritten rules?

I don't know the answers, but I think a good way to find them would be to grab a bunch of their recent romantic suspense titles and checking things like chapter length and when the MCs meet. If there's a definite pattern, that may indicate a preference for these sorts of things.

Some of this you could check without a trip to the library. The Look Inside feature on Amazon will often have a table of contents so you can see how many chapters there are in a book, and if the wordcount is around 70K that will give you an idea of average length. Reading the sample will tell you whether the MCs usually meet in the first chapter or two or not.

The rules vary based on the line you're aiming for. Your best approach is the one you're doing, to read (recent) books from the line. Another idea would be to ask people on the Harlequin Community writing forum https://www.harlequin.com/community.html

There are pretty clear guidelines here, and reading several recent books from the line of interest is also good to get a feel for it.

For the Inspirational (Love Inspired) Harlequin lines at least, not everything is written down.
For example, in the Historicals the hero and heroine cannot spend the night under the same roof unless there's a chaperone as well, but it's acceptable in a contemporary so long as they sleep in separate rooms. The hero and heroine POVs both have to have pretty much equal time in each chapter. For the suspense line, hero and heroine have to meet by the end of the first chapter.

I think so long as you read some recent books in the line you're writing for, you can get a feel for what they are looking for. If they love the story, they'll edit out little things that are not acceptable or ask you to rewrite it.

I can only talk about the Inspirational lines (Love Inspired). The Love Inspired Suspense line has a "rule" that they meet by the first chapter.
https://www.harlequin.com/community...scussion:a2e9cd29-e681-4dd2-b7e9-5d099f42f2b0

The Love Inspired Suspense books are only 55k, so they probably feel that there isn't enough time for a long lead-in to the hero and heroine meeting.