... Timing chain cover leak is very difficult to fix as there’s no gasket, and the RTV sealing is very tricky due to the 2-piece cylinder block design on SlyActiv-G engines. One member here was trying to fix the leak several times by Mazda dealer under warranty, but until today it still has some seepage from it. ...
I've had good success in the past stopping slow oil pan gasket leaks and weeps by applying RTV to the outside of the gasket, in the area of the leaking, along with tightening up pan bolts in the leak area. And, although I've never needed to do this on a timing cover, I don't see any reason why the same technique wouldn't work there as well. There's not a huge amount of pressure at the gasket, so firmly attached RTV should be enough to block the flow. At least that's how its worked for me on oil pan gaskets anyway.
That said, getting complete access to all of the timing cover gasket would be much more challenging than the gasket on an oil pan. And it wouldn't surprise me if some portions are basically inaccessible, unless the part(s) blocking access are removed. I have not removed the underbody splash guard on my Mazda yet, so I don't have any idea at this point of what access to the lower block is like from under the vehicle. Perhaps someone else here can give their hands-on perspective of how it actually is.
But for the amount of work (or $$) that would be required to remove, reseal, and reinstall the cover, I would absolutely give it my best effort before giving in and doing the much larger project. Particularly in a case like this where the much larger job might not even be successful. The steps involved for the 'patch' job are quite simple, and are well within the capabilities of most any DIYer who is willing to put in the required time and effort.
First, as mentioned in this thread previously, the entire area of the leak (including everything above it) needs to be cleaned well. Then the vehicle gets driven and rechecked, until the point(s) of the leak are identified. Then, if the leaking is confirmed to be at the timing cover gasket, the area is cleaned again, with all oil residue removed from the actual gasket area.
Once the leak area is completely clean, a small amount of RTV such as Permatex Black is applied over the gasket and smoothed out, leaving a nice solid layer covering the leak area, including overlapping the metal on both sides. After all of the required RTV is applied, wait about an hour, and then re-torque to spec (usually around 100 in-lbs) all of the timing cover bolts that are in the immediate area of the leak. And of course some bolts may also be blocked and inaccessible.
Again, it might be very difficult (or impossible) to do this work in some sections of the cover, but I'd have no problem giving it a really good try, keeping in mind that small tools might be able to get to places where fingers cant go. And for any areas which do get covered with RTV, I'd expect there to be a very good probability of long term success in stopping the leaking. And if this patching job does stop the leaking, then rechecking the tightness of the cover bolts periodically would be highly recommended.
Edited to add letting the RTV fully cure for 24 hours before driving if possible.