In my solution I use med.
Hope that's valid.
That is my solution also. Prepending is more powerful option but MED should be a viable solution also unless specified that we may not use it?
Yes, i would say in the BGP section you could mangle any attributes, as long as they don't imply you anything or you break something else (which can be the case, i saw it ).
Good luck with your studies!
Following the wording of the solution, I went for the more complicated solution : conditional route-advertisement.
With other solutions, you cannot guarantee that the upstream router is not configured with some weight or local-preference that would make the other path prefered whatever tweak you apply to the as-path or MED...
Because they are consecutive networks I used aggregation (to a /23) and an unsuppress-map on R6 neighbor 126.96.36.199.
Is that valid too?