Questions about just real three sentences.

1a. She would have a party every Saturday night.
1b. She would have a party at every Saturday night.
1c. She would have a party in every Saturday night.
Question: Which is/are grammatically correct?
By the way, if the preposition "at" is added in sentence 2#, Does it make sense or match grammar or logic?

2a The car had flipped ten times before it landed on its roof.
2b. The car had flipped for ten times before it landed on its roof.
2c. The car flipped ten times before it landed on its roof.
Question:
Which is/are grammatically correct?
In the 2 #, I greatly regard the preposition "for" here is correct by both grammar and logic. BUT I don't know whether or not it is grammatically correct?
if being in that case, is it impossibly be misunderstood?
3. Sandpaper is strong paper that has a coating of sand on it.
Question:I guess:
The "that" here is a relative pronoun , introducing a relative clause : that has a coating of sand on it. The pronoun "it" here refers to the noun "(the strong) paper".
Is my guess grammatically correct?

Question

Questions about  just real  three sentences.

1a.  She would have a party every Saturday night. 
1b.  She would have a party at every Saturday night. 
1c.   She would have a party  in every Saturday night. 
Question:  Which is/are  grammatically correct?
By the way, if the preposition "at" is added in sentence 2#, Does it make sense or match grammar or logic?

2a  The car had flipped ten times before it landed on its roof.
2b.   The car had flipped for ten times before it landed on its roof.
2c.   The car flipped ten times before it landed on its roof.
Question: 
Which is/are grammatically correct?
In the 2 #, I greatly regard the preposition "for" here is correct by both grammar and logic. BUT I don't know whether or not it is grammatically correct?
if being in that case,   is it impossibly be misunderstood?
3.   Sandpaper is strong paper that has a coating of sand on it.
Question:I  guess:
 The "that" here is a relative pronoun , introducing a relative clause : that has a coating of sand on it. The pronoun "it" here refers to the noun "(the strong) paper".
Is my guess grammatically correct?

Answer

1a.  She would have a party every Saturday night.

no "at" or "in" needed

2c.   The car flipped ten times before it landed on its roof.
coz actions are consequent and connected

3.   Sandpaper is strong paper that has a coating of sand on it.
no, "that" is used instead of "which" here, they are interchangable

Answer

1a is correct. 1b is not correct, you could say 'She would have a party at her house every Saturday night', without 'her house' the sentence doesn't make sense and it's not grammatical. 1c is not correct.

2a and 2c are correct. 2b is not, you could say ''The car had flipped for ten minutes before it landed on its roof" using 'for' as a synonym of 'during'.

You are right about 3, it's correct and your explanation is good.