Hi. Could these sentences be used in a typical letter of reference or a letter of recommendation?

1. I think the word in parentheses is optional. Does the present perfect tense here indicate he is still with the company? Then, I think the tense is wrong since this should be a letter of reference for his/her job.

(made-up sentences for asking grammatical questions)

He has worked for our company for thirteen years and (has) proven himself reliable and punctual, contributing greatly to high sales figures for his department every year during his stay with us. His department's yearly sales figures have been on the rise since he joined our company as department supervisor thirteen years ago, when the department had been showing stagnant performance levels for some years.

2. Is the underlined present perfect tense correct as sentences that supposed to be part of a letter of recommendation? I think the present perfect tense here indicates he has finished his studies at their school some short time ago, but not a long time ago. Do you think I am correct? Also, do you think the present perfect usages (uses?) in no. 1 and here are the same in terms of what they are supposed to indicate (if you know what I am trying to get at)?

(made-up sentence for asking grammatical questions)

He has attended our school for two years, and during that time, he consistently achieved good grades on all of his subjects which he undertook every school term, while being a valuable member of our basketball team, which has performed magnificently over the years, bringing many trophies.
Language is ego-centric: the language we use reflects our perspective on a situation/event.

Say, this is a headmaster writing about John. What is this letter revealing?
"He has attended..." : John is still being regarded as a student at the school

but then the headmaster changes to past tense: "...during that time, he...achieved good grades"
Something has changed/ended, otherwise he would have said, "...has achieved" /"has been achieving good grades, such that we fully expect he will be admitted to Harvard."
As it is, being in the past tense, it indicates he will no longer be achieving good grades because he won't be taking exams - he must have graduated...
He has suffered brain damage, and can no longer be expected to be as bright.

The headmaster writes specifically,"...during that time, he...achieved..."
He is referring solely to a previous "2 years", and that this has come to an end - "achieved" -, otherwise he would have written, "...has been achieving", indicating that his achieving good grades is still an ongoing process - he is still a student.

The writing gets confused in the last sentence, when the headmaster ceases to be talking specifically about John, and starts lauding the school's basketball team! It would have been better to have written:
"...whilst being a valuable member of our basketball team and performing so magnificently over the years that his team won many trophies."

The basketball team will still be there after John has left, and so "has performed...over the years to now, and we expect this to continue; but as far as John is concerned, it is past tense: he performed...

Referring back to 1.
The actual inclusion of 'has' in 'has proven' is optional, but the tense is still Present Perfect.

The Present Perfect tense is correct IF he is still working for the company but applying for another job elsewhere.
If it is a letter of reference to be added to one's resumé, then it should be Past Tense.
TerryxpressThe Present Perfect tense is correct IF he is still working for the company but applying for another job elsewhere.

Hi, Terry,

Isn't this dependent on adverbials?

For example,

He has worked for X/He has worked for X before/He has worked for X in the past= These can all suggest that he worked there in the past.

He has worked for X for many years= In this case, yes, he still works there