Hi Odmara,
I was just reading a comment from a class mate that pointed out the importance of developing a tool that is sensitive and specific to assess cognitive function, maybe you are undertaking this huge task.
Regarding your questions, I know that Lumosity has shown to improve working memory and attention primarily in older adults.
Your question regarding the knowledge of when to start or periods of higher susceptibility are not yet clear, but I believe it is being explored. I think in primary brain tumors is a little difficult to establish this time-points since the speed of cognitive decline varies depending of a number of factors (type of tumor, location of tumor, size of tumor, age, therapy, symptoms at diagnosis, initial cognitive function, etc), but it I think it is worth to explore this area to be more accurate when recommending this type of intervention.