+ if there are many stacks with the same speed, then wouldn't all the attacker's units move faster than all the defender's units, in other words, which is more important: a) or b)?

Let's consider your example:
7 stacks of Trogols attack 7 stacks of Imps (Trogols are either upgraded, belong to Shakti, are on their own territory, or have an artifact that gives +1 speed; in this case, they will also have a speed of 5, like the regular Imps).
The top stack of Trogols moves first, then the top stack of Imps, then the second stack of Trogols, and so on. The bottom stack of Imps moves last, and in the next round, the top stack of Trogols moves again. But if, for example, one of the Trogol stacks uses the "wait" command, then it will move last in the first round, and in the second round, the top stack of Imps will move first...

reply to the next post

(usually with cavalry or a similar unit) moves closer at the end of the round and attacks first at the beginning of the next round!

This is only possible if the cavalry has a higher speed than all your units. Even if one of your stacks has the same speed as the cavalry, the cavalry will still move last in the first round (regardless of whether it delays its turn or you both delay it; in the second and any subsequent round after that, you will move FIRST!
There are many ways to change the turn order, but the rule of movement remains constant!!! ... it's just that if the opponent is also "monitoring" the turn order, they can use countermeasures to prevent you from moving as you want, but any situation can be calculated... and no matter how many times you try, if you follow everything sequentially, the situation "what if in some round, a different stack moves first" will never arise.

Added 10 minutes later

I know this rule.

It's clear that you don't understand the rule.

which is more important: a) or b)?

"a)" is more important (that's why it's "a" :) ), but it's only important for the first round, or rather, only for the very first action in the battle; after that, everything follows point "b," and the principle of "left moves - right moves."