FWIW I once owned an Italian motorcycle which used multi-pin versions of connectors that looked like Tamiya ones, but made by Molex. These were poorly positioned on the machine so collected all kinds of crud like it was going out of fashion. However it turned out it didn't really matter where they were placed, road spray still got into them, and in the winter that meant they would start corroding (because of road salt) unless otherwise protected. I have encountered similar problems with various electrical connectors on bicycles. If you never ride on damp/gritted roads there is less to worry about in this regard.
In the OP's position the extra light is presumably to be run in parallel with the usual rear light, so it arguably isn't the end of the world if the connection isn't perfectly reliable. Given that the wish list is to be compromised to some extent, maybe waterproofness is worth compromising on to better arrange the other qualities desired?
With this in mind I'd favour either
a) installing a socket in the existing rear light housing (e.g. for a small jack plug) or
b) installing trailing leads with trailing bullet connectors on them near the rear light.
In both cases water resistance and corrosion protection would be obtained by using something like waxoyl on all the exposed metal parts. I have found this is adequate for otherwise unprotected dynamo connections.
The other thing is that some B&M rear lights have two types of connection to them; e.g. 2.8mm blade connections
and bare wire connections. This makes it easy to install a second rear light in parallel, using the second set of connections.
I don't remember if the secula has a second connection on it or not, but if not then a different main light (with two connections) and using the secula (with a suitable bracket) as a 'bag light' might work as easily as anything else.
'Toplight line plus' has two types of connection as described
'some manufacture of brackets required'
hth
cheers
