Colloquially you can use either, but if you were to say 'in the mall' you would likely be referring to an object. There's a bookstore in the mall. If you said 'at the mall', you would presumably be referring to a person. Where is John? He's at the mall with his girlfriend. Of course, 'mall' can mean the physical building complex, in which case anything can be 'in' the mall, but 'at the mall would almost always refer to a person.
The sentence is gramatically correct.
yes.
Everybody has is correct.
Yes it is.
No 'It's for you' would be though
It's a weird sentence, but it is grammatically correct.
Yes it is correct to say "Have a wonderful Holiday."
No because and is a fanboy and it would not be gramatically correct
i welcome you TO Delhi.
Yes, there is a verb and and a noun and they are in the same tense therefore it is gramatically correct.
You should say for your convenience it is *gramatically correct-*just means correct
Yes.