SeaMonkey point realeases appeared in the repos faster when 12.2 was the current version. After release of 12.3 they started releasing SeaMonkey only when a major release happens.
You need not install from tarball. All you need to do is download the binary from below link and then extract and use executable inside it. It will automatically use the exiting profile in your home https://download.mozilla.org/?product=seamonkey-2.17.1&os=linux&lang=en-GB
You could download just the seamonkey .rpm from the OBS Mozilla repo and install it by hand.
And yes, SeaMonkey 2.17 is planned to** be released as online update**, but I don’t know when. AFAIK there were problems with building it, that’s why the update is not out yet.
A couple of weeks ago, OBS had problems building Mozilla projects for their memory requirements, and they kept failing. I’m guessing they might be waiting for the upgrade to OBS, before going again.