Well. It's a pretty unbalanced squad. If I can give you an advice, use teams with almost the same total power. Eve if it's not really revelent of the true power of a team, it can give you an idea. Espacially at the begining.
Here, your first team is pretty good with 2 5* and 3 4* but your fourth team is really weak. During a mission the first team can easilly rush through the stage and reach the end but teams 3 and 4 will certainly die quikly and don't make their job. It's annoying if you whant 3 medals on a stage.

About your questions:
1: at low level, just use your best girls and try to make teams with almost the same speed/total power. At higher level, when you will have enough girls, try to teams them according to their abilities and roles. For example, Toad Lily have ad attack boost (+12% attack for herself and 3 allies when she is evolved). She fit well in a high damage team. Yulan and Di Yu are better for defensive teams.
And don't forget your allies. Try to find somes with higher level and really good teams. A powerful ally is often really helpful.

2: Like I said in 1, abilities are certainly the most important (at least, it is for me). Skill and skill rate are also important for skill spam or high damage team. And the stats but it's obvious I think.
But the most important for me is the synergy between the member of a team. A good team is not a team with the highest total power but a team where girls' skills and abilities work well together. Give equipment to your girls also improve her stats. so, 5* event girls with multiple equipement slots are good.

3: All the speeds are important. For most of missions you need teams with almost the same speed. But for somes (espacially in event missions), you will need teams with really high speed and teams with really low speed.
Try to get the 5* Oncidium. Her evolved form give you a +100 speed bonus for the team. It's really helpful.
Also, try to get 4* Lilac. She ignore spider web and wirlwind nodes that change your speed.

4: Idk

5: All I say previously. Also, there is some threads here to learn how to play efficiently like how you should use manyu or how you should optimisethe use of stamina and level up.

Can you expand a bit on the 'synergy' part ? How does the girl's skills and abilities work well together ?
Thanks.

Many girls have the same abilities or close abilities. If you put them in the same team, it will be more efficient.
For example, girls like Convalaria, German Iris or Gerbera have a bonus to solar blast damage. If you make a team with her, you willget a solar blast bonus of 65% (if I'm not wrong). Add girls like Bipinna Cosmos and Strawberry Candle who have a shine crystal drop bonus and you get a team with a really strong Solar Blast they can use more frequently.
Take girls like Rose, Snapdragon and Anemone. They all have a counterattack ability based on their defence. Add girls with defense bonus and you will get a good counterattack team.

For skills, it's the same thing. Pair girls like Convalaria or Toad Lily with a 3 target skill and pair them with Cyrtanthus and Justicia (who have a x1.2 skill rate bonus based on affection for the whole team) and you will have a nice skill-spam team with good multiple-target skill. As (th girl you can add one like Iris to get even more damage bonus (Toad Lily and Justicia already have one).

After that, you also need tomake teams with what you have. If you don't have any counterattack girl, don't even try to make a counter team.

Another example, based on your team, your Bride Cactace will work very well with a girl like Anthurium with her +22% damage on &st turn. Since Bride Cactace already give a skill rate bonus.
Another one, the current reissue Girl, Moonvine will give you a x1.2 skill rate bonus on the 3rd turn. The next reissue girl, Tachibana do the same thing on her evolved form. And the next next reissue girl is Star Lily with her +15% damage on 3rd turn. These three vork wery well?

That almost what I mean when I spoke about synergy between girls. And it's also almost the main topic of this thread ^^. You certainly can found a lot of ideas in it. But it mainly for slightly more advanced players with more girls. For a beginer, already try to do with what you have and good luck for gacha ^^.

Originally Posted by maotd

Another example, based on your team, your Bride Cactace will work very well with a girl like Anthurium with her +22% damage on &st turn. Since Bride Cactace already give a skill rate bonus.

Bride Cactace has a faulty description though, her skill trigger is only for herself and not the entire group.