Update...by the end of conquest i actually managed to do the 4 first sectors...so i can answer myself...
1) Hard provides more rewards as long as you have an effective GL (with ultimate). I used SEE to "unblock" hard nodes...i would get ONE star, but move trough,
2) More GLs are good, but not NECESSARY. GOOD TEAMS and BUFFS help getting three stars on many nodes, BUT the "tasks" are mostly impossible for anyone with an "incomplete" roster. HARD "needs" lots of reliced teams to finish, including "recent" teams that free players wont have anytime soon. BUT an unfinished HARD is better than a complete MEDIUM.
3) SEE can one star most nodes. I did fail a couple, but could go around or use something else. I DID NOT FINISH so not 100% sure it can be done, but i got to sector 5.
4) VERY GOOD Jedi, Sith, GR, Empire, ITroopers will clear nodes in HARD; If improved i guess they will be even better...so i will RELIC a few of them up to 5 (or 7 if possible). Way cheaper than getting more GLs.