Mostly you are correct. I noticed a few things though.
I have been walking in the rain.
This is definitely the correct answer!
I have read all morning.
This should be "I have been reading all morning." Because of the context that reading holds, it doesn't make sense to use the present perfect simple. Either the reading is still going on (present perfect continuous), or it was in the past (past tense like "I read all morning").
How many pages have you read?
Yep!
I have read 50 pages.
Yes! Now the present perfect simple makes sense. In a situation where you accomplished something- like a certain number of pages- this tense can be used.
Lina have eaten three ice creams!
The tense is correct, but it should be "Lina has eaten three..." Because Lina is third person- someone we are talking about- the verb to do is conjugated as has, not have.
In addition, in the US, we do not ever make ice cream plural. We might say "three ice cream cones" or "three scoops of ice cream" or "three bowls of ice cream" or "three ice cream bars." But never "three ice creams." I am not sure if this is only an American thing? Unless you know for sure that British English/Australian English/whichever other form you're learning uses this plural, I would default to using a different word.
I have been working in the garden.
This is also correct!