1 - I'd save them for at least the bigger four (SS/AotIL/DP/RR). Save them until you're nearing the final couple of levels. It will save you a ton of money
2 - Currently the ios system has it where leadership stats are pretty crazy, if you are ios the following bonuses occur. 20% for Guild leader and 15% for Sentinel in your case. For android, it's not implemented like that yet (hopefully wont be)
3 - You could do that, however it would probably be smarter to just get the guarunteed 50ep fusions (basic water/basic earth) and level your current bigger four that way. But really, I think it'd be best if you saved them for guild related quests and only used for extremes like legendary or the final couple of levels on an epic (at minimum)
4 - Can't answer for I haven't done it yet! :)
Hope that helped