you seem to have several questions here so I shall go through in the order you numbered them.
Most micros will indeed run C++ (assuming the manufacturer or an open source project provides a compiler back-end), however you have to be wary of a number of drawbacks. C++ Is less deterministic, as in, it provides a significantly higher level of abstraction, which one likely does not want an a resource constrained embedded system, and by and large it is not needed either as embedded systems are rarely powerful enough to usefully run the enormously complex algorithms that warrant a high level language like C++. It is also likely to cause a wide range of hard to track bugs, given the difficulty of debugging code from an embedded system having bugs which are simple and easy to trace are very much nicer. However very importantly, the C++ standard libraries are enormous, they will use excessive ram and very likely waste much of your limited memory space. Thus, even if you do use C++, you wont be able to use any of the techniques that make it powerful.
Simply, I have not used it, however like any RTOS, it is useful if you want a slightly higher level interface, however for a micro the tiny size of the MSP430 it seems overkill, I cannot imagine you doing anything on there that warrants an ROTS, if you need multitasking it would be better to provide simple cooperative tasking yourself.
Unfortunately I have not used that platform either, however given it is based on wiring, my guess it that it does not provide high levels of hardware specific optimization, if you want that I recommend using it for the bulk of your code but make calls into lower level libraries when needed. Beyond that however, it does provide a lovely, self documenting interface, I strongly encourage you to try it. It will also make your code many times easier port if you switch to another micro later (many systems from many companies provide wiring bindings).
You really answer this yourself here, it could be very powerful but is still very immature, I would avoid it purely because of this lock-in until it becomes more mature, then it is worth re-assessing.
Java works nicely on more powerful ARM chips, that is the only place I have seen it in wide use, and implemented fairly efficiently in a micro (ARM provides hardware assistance specifically for Java). Other than this Java is a poor fit for the micro world, at one point it appeared it might go somewhere but this was largely unrealized, for now C likes are the way to go for smaller micros.
Unfortunately there is not really a huge amount of choice other than C. My best recommendation is using higher level libs like wiring. That gives you slightly nicer interfaces without killing efficiency, otherwise there is little point in using a tiny micro if you need high levels of abstraction.
In summary, C does a fairly good job here, I do not think there has been any motivation or effort to make a good replacement. And frankly I largely feel this way too, C never became a bad language, it is still well suited for small systems for the same reasons as it was before. It provides power, efficiency and predictability.
I hope this helps somewhat, if you have any queries please comment me and I will see what I can do to help.