Fill in the Blanks

 'By next month' means the action will have been completed by then. 

 There is no continuation involved to warrant the use of 'ing'.

 At that moment he was in the process of reading. 

 Obviously she 'had completed' work before she went to play.

 At that moment we were playing. 

Meeting had to be in the past, i.e. before would have invited you . The antecedent in time to would have invited you is 'had'. If I had..., I would have...

 'For two hours' indicates part completion (have been), working indicates continuation. 

Since the condition is in present tense "he gets" ,Hence the modifier "will bw" in future tense

Hope is for future. 

 'Next October' is in future (will). 'For five years' indicates part completion (have been). 'Living' indicates continuation.

 'When she gets' indicates future (will), 'for two years' indicates part completion (have been), 'studying' indicates future.

 'By that time' indicates future (will) as well as time limit for completion (have finished). 

'Acted up' means 'behaved in an annoying way'. This meaning fits in the framework of the meaning of the sentence.

'Adds up to' means 'indicates something'. This meaning fits in the framework of the meaning of the sentence.

'Fell about' means 'laughed uncontrollably'. This meaning fits in the framework of the sentence.