В офісі is correct too!
it should be actually only one correct because of two consonantes "o"
You're right, it's easier and more "natural" to say V ofisi than U ofisi
Does this makes a difference when translating? Would it change to "... at the office" or would it always be "... in the office"?
I use the word 'часом' for 'sometimes'.
We never used 'інколи' in our family. (It 'sounds too weird'.)
I wouldn't say it's weird to use "Інколи". It's OK.