Don't need the randomness, we're trying to fix the map to remove snowballing, right? Let's focus on that.
5x is too much. Triple is plenty. And passive xp. Gotta have that passive xp.
Reduce minion rewards significantly since we now have passive up. Also reduce champion kill bounty by about 2/3. Again, anti-snowball.
Now fix the global mp5 problems. Either give a strong passive aura for it, or go the full distance and grant the URF manaless buff without the CDR.
Try that. It should make the experience much more pleasant and less toxic.