Deciding whom to emblem can be very difficult… Regarding your options, I would chose the following:
Alasie vs Lianna: Alasie!
Green has many great heroes and a lot of them are rangers unfortunately. Lianna is already pretty strong but will struggle to one-shot many near-full 5*s anyway. Giving her a few emblems doesn’t change that.
Alasie has some additional utilities, namely the mana slow down and - a lot of people forget about it - the elemental link. Besides Magni and Lepus, Alasie is very important as a fast heavy hitter and can help you to form a strong blue core!
Poseidon vs. Elena: Poseidon!
Not much to discuss here, Elena only shines because of her sky high Atk and is often used in Titan fights but poseidon is a slightly bulkier version of Joon and can be very valuable in certain match-ups because of his unique skill. Sure he doesn’t profit that much from the fighter class skill but boosting his stats can transform him into a big thread when set in flank oder corner position!
Justice vs. Richard vs Frida: Frida!
Frida is very important against red titans and a must-have for high scores especially since you don’t have Arthur. She is also very great in blue stacks when put together with your alasie and maybe Grimm wreck havoc among your opponents! Justice is OK but still not very dangerous as a tank, a few emblems wouldn’t change that. Richard is pretty awesome with Frida but not a game changer on his own.
Sartana vs. Onatel: Onatel
This one is really not that easy. Both are great on offense and defense. But Onatel hitting three opposing heroes will profit more a little bit from jinx. She can be very difficult to kill after just one cast of her special and a few emblems can make her even more beasty!
Spreading emblems amongst several heroes?
I would advice against it! 5*s need an enormous amount of resources and few knots won’t make much difference in power anyway unless you rather aim for the class skill and not so much for the stat improvements. For some heroes a few extra Def, Hp or Atk or unlocking the class ability may already help on rare occasions but generally I would prefer one of the two following strategies:
a) Emblem your game changers!
This will help you a lot against titans and increases the effectiveness of your raid defense!
b) Emblem your key 4s!
4s will progress much faster through the skill tree. You can try to cover weak stats or make an already tanky hero like BT a defensive monster! So many possibilities here!