Given that your sleep therapist said, "there was a major leek
(sic) in the mask that had been getting worse
", you should do something about it. First off, get the free software offered on this site and see for yourself what the leaks are measuring and whether or not they are high enough to be affecting your treatment. Let's face it, if the leaks are at such a high level as to render your treatment ineffective, then you're going through all the inconvenience of wearing a CPAP mask for nothing.
If your mask is more than a year old, as you have stated, you're way
overdue for a cushion replacement. The Mirage Quattro is a good quality mask but it doesn't last forever. If the hard parts are still in good shape, then replacing the cushion and possibly the headgear (the strap) will bring it back to the performance level of a new mask. The cushion is available for 50 bucks +- and that's where you should start. Check the headgear for its ability to stretch at least 2" when tugged firmly from end to end. If its still that flexible, then it'll be OK to use unless you can see that it's frayed and in danger of breaking. Frankly, any fool (me for example) can tell by the general appearance if the headgear need replacing but the stretch test will confirm that.
Since you know that your jaw is dropping down during the night, do as I do, open your mouth slightly (or even more so) before installing the mask to your face. That alone could help the mask maintain a better seal all night long. Having a major leak, as you have been told, will usually made enough whistling or farting noise to wake you up. Getting back to sleep after that can be a problem since your subconscious mind is telling you to get out of bed, you've slept enough. In this case the subconscious mind is mistaken.
A chin strap will only work if you are fully able (at all times) to breathe comfortably through your nose. If you have a tendency to be a mouth breather due to some nasal problem that isn't temporary or easily fixed, then forget the chin strap and work around the problem as I have suggested by using a full-face mask (as you are doing) and dropping the chin slightly when installing the mask.
Bottom line is, don't blame the messenger, get that new cushion and be thankful that someone pointed out that you needed it.
I will add that mask liners are great for people who have hard to fix leakage problems. You can buy them or make your own out of soft cotton. It's not rocket science and it works very well. I make mine from my old tee-shirts that have been discarded because Mrs. surferdude says they need to be in the rag box. I made my own template and cutting them out is quick and easy. I may buy some professional ones someday if I expect company and want to show off but in the meantime these home brew ones are doing just fine.