mail client for Mac with these characteristics?
I would suggest taking a look at Nylas Mail. It's open source, has a wealth of plugins and themes on GitHub, and is relatively "fast and robust." As for your other criteria:


It allows for messages to only be marked as read manually, as can be seen below:


All shortcuts (including "mark as read") are completely customizable through JSON. For instance, one could have the r key mark messages as read by putting the following in one's keymap.json:

{
    "core:mark-as-read": "r"
}

Nylas supports a dual-pane mode (similar to Apple Mail), allowing one to preview a message without having to open it in a separate window.


Unfortunately, while Nylas does have built-in spell-check, it does not use Apple's native solution.

Nylas also boasts a host of other features, including read receipts, link tracking, message snoozing, and templated quick replies. There is a Pro version, but all of these features are in the free version (which I use).

For the record, I am not affiliated in any way with Nylas—I just like their mail client.