To me, 'cho ̣hạnh phúc' would make more sense. Probably this is more idiomatic, but just sayin'.
.. I'd choose vao, but just sayin'.
For sure--I don't understand 'của'...as it nearly always indicates possession or inclusion.
think of it as "of"
You don't get to have this both ways. Either Sự is used or it's not. You don't get to do it both ways.
that was a tricky one, whew! but I got it!! XD
Again, why not 'cho'? There is no possessive here. And this is not suggested in the hints either.