When he filed the motion for an extention for responding it should have stated how long he wants it extended for, you should have gone to a hearing and talked to the judge about his motion for an extension and told the judge why you do or do not agree with his motion. The judge would make a ruling on a timeline for him to respond.
As for the mortgage, who is living in the home? If he is living in the home that it would no doubt be his responsibility fto make sure he pays for his own residence, sometimes a court will rule for him to continue to make the mortgage payments even if he does not live in the home due to cercumstances such as you are not employeed or are a stay a home mom and rely on him to keep you.
All of these motions add up to either a trial or a divorce and yes the more motions that are filed the more costly the divorce becomes and the longer it takes. The motions are just put in place to temporarily make things work until a final divorce decree can either be reached between the two of you or court ordered by a judge after a divorce trial. Hope this helps