Navio2 | C and Python sensor examples for developers

A few answers for your points:

1. Melodic is supported on RPi 3 B+ now with Raspbian Lite. You might need to do a little apt-repo jockeying, but my team was able to get it without too much trouble.
2. There's nothing stopping you from using C++17; just update your CMakeLists.txt files as appropriate to add -std=c++17 to the CXX_FLAGS
3. To the best of my knowledge, ROS Melodic at release supports OpenCV3. It might take a little while for OpenCV4 support to be made available -- I still need to check on the project to get the latest news.
4. I haven't actually looked at the Python3 support yet. I think its being deferred to ROS2.

That said, I wouldn't say that ROS2's immaturity is a good reason to disqualify it -- I know it's going to take a long while before there's enough support and all pre-existing ROS packages are ported to it (that's why my team didn't want to consider it).

In any case, I'd still consider if any of the algorithms you're planning on implementing really need OpenCV4 features, or if using that version is simply a "nice to have." OpenCV3 isn't really that old, and from what I can gather, handles everything state-of-the-art without making the algorithms bend too much to the API.

Update 28-Dec-2018

Doing some cursory searching, it looks like ROS Melodic does have Python3 support (see this question): ROS has to be built from source with the environment variable ROS_PYTHON_VERSION=3 set (see also this)