Both of your original conversations look fine. And yes, you can combine your responses into one. Here are some possible candidates.
For example #1:
"Yes, I have already sent that email (out) today."
"Yes, I had sent that email (out) earlier today."
For example #2:
"Yes, I have already sent that email (out) yesterday."
"Yes, I had sent that email (out) yesterday."
"Yes, I had sent that email (out) last week."
"Yes, I had sent that email (out) when I was in Japan."
NOTE: In your excerpt, there is "You CANNOT use the Present Perfect with specific time expressions such as: yesterday, . . ." <== But that is not really true. Your excerpt is an overgeneralized "rule" which is not always true. There are some conditions when past-time adjuncts (such as "yesterday") can be used in a present-perfect construction. For instance, there is a counter-example in CGEL, page 144, [13.ii.a] :
- "We've already discussed it yesterday."
ASIDE: Here is a post with some info on the present perfect construction (though, unfortunately, it probably isn't directly related to your question) : What's the meaning of this present perfect sentence?
NOTE: CGEL is the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language.
When talking about things in the past, the past perfect happens before the simple past.
In your example
If someone feels that they had never made a mistake in their life, then it means they had never tried a new thing in their life.
had never made and had never tried are two actions which were started and ended in the past compared with how the person feels (present). The equivalent would be
If someone feels that they never made a mistake in their life, then it means they had never tried a new thing in their life.
where never made is nearer in time than had never tried since one would have had to try before failing.
I had called you
I had gone to the wash room
could individually use the simple past
I called you
I went to the wash room
with similar meaning, however, using past perfect sets up a scenario for something in the more recent past
I had called you before I went to the wash room
I had gone to the wash room before I called you
To answer where you were when they called:
I had gone to the wash room when you called me
means you were in the wash room (past perfect) when the person called
I went to the wash room when you called me
means you heard the phone ring then went (simple past) to the wash room
Best Answer
In sentence a, had convicted is past perfect: you only used this to talk about something that happened before some event in the past.
In sentence b, convicted is simple past: this is used to talk about something that happened before now.
Sentence b is the right one to use in this example because there is no other event in the past. Note that in the UK it would be normal to put the definite article before court, and the correct preposition to use for convict is of: