Maybe some consideration should be given to a "Time Triggered" OS
Link here to a free book
http://www.tte-systems.com/books/pttes
Also they have quite a few seminars on YouTube.
Their hybrid system is a co-operative scheduler with just one interrupt.
This system gives a lot of predictability (in terms of time) and by allowing just one interrupt, all the issues relating to pre-emptive multi-tasking are virtually bypassed.
Their system is often used in safety related applications, ie car, planes,
Here is a link to a port to Arduino (well a beginning anyway)
Just my 2c
Kim