Finally, someone asking the real question. It has been asked before, but not until full blown questions like yours.

I thought most of us figured it out by ourselves. I have get used to this, so no more annoyed by the prompt anymore. The following are translated from my own documentation in the other forum, with some modification.
The basic principle :
1. Automagic identifies each element (trigger, condition, action)
by its name. So, you
can't have two different elements with the same name but doing different things. One of them must be renamed to something else.
2. If you
don't change default name, that is the name of the element. I usually leave it default as well, even when sharing the flow
3. Default name usually created using the
element name + part of the option. Example : Sleep with 1 second and keep device awake, will have default name
Sleep: 1s (keep device awake). This will vary depends on the element.
4. Some element don't have additional option, example :
Clear Automagic Log. This element will have the same default name all the time.
5. Some element have too many options,
but only one or two options participate in the default name. Example : Action
Speech Output, has a lot of option :
Voice, Audio Stream type, Text, Speech Rate, Pitch, Synthesis type and Options; but the one used in default name only
Text.
6. The message prompt only appear when you have 2 elements doing the same thing or reused several times
Reuse/Overwrite
a. You made a new action
Speech Output, Voice
English, Text :
Hello. Default Name :
Speech Output: Hello
b. You open that action again, change the Text to
Hello World. No prompt, unless you have ever made that action before. Default Name :
Speech Output: Hello World
c. You create a new element, Speech Output, Voice
Deutsch, Text :
Hello World. The default name will be the same as above :
Speech Output: Hello World. So when you save, there will be prompt to
Reuse/Overwrite.
- Overwrite = Replace the b with c, so now the first and second element both using Voice
Deutsch. (This replace all option, Voice is only example)
- Cancel = Cancel, so you can re-edit the name
- Reuse = Reuse the b option, Replace c with b, so now first and second both using
English, even though just now you edit the voice to Deutsch.
In most cases, I usually choose
Reuse, because most of the time I reuse a similiar action from other flow. But when you encounter something like this, where you have to speech output the same sentence in two different language, you should rename one of them to be different, example adding
Deutsch at the end of c element. (will be explained in Change Name)
This Usage / All Usages
If you choose
OverWrite (c replace b) or
Reuse (b replace c), then you will have 1 element used 2 times. When you edit one of the element again (c), it will show the same as your first screenshot.
This action is used 2 times. Click for options... If you click this blue bar, you will see the same prompt, stated which flow use this same element. The option
- This Usage : If you choose this, anything you edit to this element will be separated from the b. But if you don't change anything, it will still the same as b. So blue bar will appear again.
If you change the option
Audio Stream Type to
Notification, but the default name is still the same (you don't change the Text
Hello World), then you will see the same prompt for
Reuse/Overwrite. Same case for the first abc above.
- All Usages : Choose this and anything you edit to this element will be applied to every element (
so b will change as well even you edit only c). Blue bar will stay, since b and c still the same.
Better use This Usage
Most of the time if you need to change something, you will just use
This Usage. For example I just copied the Action
Speech Output: Hello World, and wanna change it to Text
Welcome. I would choose This usage, so now b has
Hello World, but c has
Welcome.
If I choose All Usages, both b and c will have
Welcome.
All Usage can be dangerous and possibly ruin your other flows. Example you have sleep action 10 seconds used 10 times in 10 different flow. Now you want to use sleep 1 second only, you wrong tap
All Usages. Now all of your other flow will become sleep 1 second as well. I don't know the consequences, but it can be disastrous, all flow logic messed up,
FUBAR. You can quickly fix this by changing it back to 10 seconds and tap All Usages again, but only if you remember the previous option. For Speech Output which has a lot of option, that can be missed easily. So,
it is better to stick with This Usage in most cases. If it is the same element you refer to, there will another prompt to ask about the Reuse/Overwrite.
Change Name
Back to Speech Output again. Just now b and c is the same already, using
English Voice and Text
Hello World. Now you open c and change the voice to
Deutsch. When you save, you will see the prompt just like your last screenshot.
- All Usages : the same as above, b and c now will changed to
Deutsch.
- Change Name : is the one you need for this kind of same default name but different option. It force you to add something to the default name to make it different. In this case, you can add
Deutsch, the name become :
Speech Output: Hello World Deutsch.
Now b (
English) and c (
Deutsch) can exist together since c has additional name
Deutsch at the end, b and c is different now.
Probably it is quite confusing to read this. You should try it out by yourself. Soon you will get used to it already.
==============================
Now I will answer your numbered question.
1. Yes,
All Usages will change all other Sleep(1000) to Sleep(2000)
2. You probably copy the action or add action and choose directly from select an action; not using new.
If you use new, this prompt won't appear. I always make it as a habit to choose new if I am going to make a completely different script.
3. That blue bar will always appear when there are same elements being used several times. Typically will appear on elements with no option, such as debug dialog. There is nothing you can do with that, it is by design. Unless you have time to change the name for each element usage, which is too troublesome.
4. As explained above. When you create new element with default name same as before one, you will be asked to
Reuse/Overwrite. Most of the time, just
Reuse. It is better to be wrong for this element only and re-edit the default name for this one only (to make it different); rather than choose overwrite, and change all of other usages to current one.
It can be the same disaster as All Usages.
5.
Change name indicate you have two element with same name but doing different action (as above, b is
English, c is
Deutsch, but both has same name). One of the them must be renamed to something else, just add some tag to the name to indicate the difference.