Your grammar book is wrong: the present perfect is a present tense and is not used with temporal adjuncts which do not include the present. Last year without a preposition or a definite article designates the year before the current year, so it excludes the present, and you should not write
∗Lots of people have helped us last year.
It would however be OK to write this:
Lots of people have helped us since last year.
And the last year is the year-long timespan which runs right up to the present moment, so this would be OK:
Lots of people have helped us during the last year,
and this, omitting the preposition, would be natural in speech:
Lots of people have helped us the last year.
You should use the appropriate tense at each point. You start talking about the summer in the past tense. No problem here.
This summer was horrible, I didn't even get to go anywhere.
Next you change to the present perfect. That's not an error. But the use of the word struggling is awkward, it looks like translationese. Better to use past tense for the accident and present perfect for your current state:
My brother had an accident, and we've been helping him recover.
"B" can also use whatever tense is appropriate. First sentence is about the past, so past tense. Second sentence is about the present, so present tense. The "I'm loving" is casual (continuous tense of stative verb), but I think acceptable here
We did loads of things and I had lots of fun. Like now, for example, I'm in New York! I'm loving everything about it.
A sentence can have mixed tense:
Yesterday I went shopping, as I will go to a party tomorrow and the bag that I had bought to take with me was too small.
Each finite clause has its own tense, and while jumping around tenses too much is confusing for the reader, and bad style, it is not incorrect.
Best Answer
He is separating himself from the action. He is admitting he did the action but also admitting it was wrong. He is stating disbelief that he would ever have done the thing he did. Could means 'possibly' in this usage.