There is a straight question and no straight answer (like YES or NO)
Please explain how to treat the note: "excluding racket". Why is that "excluding racket" there at all?
because leaving the court to get a racket is treated differently than leaving the court to get an article of clothing, shoes, etc. WHY? the USTA doesn't explain....perhaps this is your question?
Does it mean the player has unlimited time to get a replacement or it means player has no right to seek the racquet outside of the court?
No. USTA Comment 29.2 answers this question.
There was no question about replacement of anything else but the racquet. So, why the USTA shifted the subject?
The match can be suspended for 3 minutes by umpire if player needed to replace anything besides of the racquet. It is known, but it does not have any relation to the subject again.
Your statement is incorrect. See USTA Comment 29.2 for the USTA of 'reasonable extra time'
Talking about Pro and Junior tennis:
There is no umpire on court in more then 50% of junior matches (or even 90% of the time during the match).
Who does suppose to make a decision about suspension or penalty points in such case?
I'm not against of your point of view. I'm not satisfied with the way the USTA answers the question.
I want to see – Yes or No in first place and then any comments to possible penalties or any other details.
How to calculate the penalty points?
USTA Comment 4.2 answers this.
Does every 90 seconds of delay add another penalty point?
no. change '90' to '20', and you are correct.
Who is in charge of tracking of the delay time?
the official
And since there are "excluding racket" it all should be addressed specifically to replacement of the tennis racquet case.
USTA Comment 4.2: May a player who breaks a racket or a
string in a racket leave the court to get a replacement?
A player who
leaves the Court to get a replacement is subject to Code Violations for
delays under the Point Penalty System. Rule 29b permits a player
“reasonable extra time” to leave the court only in those cases where
“clothing, footwear or necessary equipment (excluding racket) is
broken or needs to be replaced