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.