Formatting and comments can be added using the openxlsx package. For example:

library(openxlsx)

# writing the data to a new workbook
wb <- createWorkbook()
addWorksheet(wb, "Sheet1")
writeData(wb, 1, df)

# apply to all cells with NAs:
apply(which(is.na(df), arr.ind = T), MARGIN = 1, FUN = function(cell) {
  # overwrite the missing value
  writeData(wb, 1, 
    sample(1:3, 1), # your interpolated value (here a random number)
    startRow = cell['row']+1, startCol = cell['col']
  )
  # highlight the cell
  addStyle(wb, 1,
    createStyle(fgFill = 'orange'),
    rows = cell['row']+1, cols = cell['col']
  )
  # add a comment to the cell
  writeComment(wb, 1, col = cell['col'], row = cell['row']+1,
    comment = createComment("Filled in with random number")
  )
}) %>% invisible

# view the workbook (there's also a function for saving it to disk)
openXL(wb)