Incapacity benefit does not entitle anybody to mortgage repayments - which is why most people take out insurance for their mortgage in the event of ill health. Incapacity benefit is not means tested - it is available to everybody who is too ill to work
Help with mortgage repayments may be available if you are receiving income support, but this applies to the interest part only. It is paid for a very limited only.
You can of course get both income support and incapacity benefits, if your circumstances tick the right boxes.
Income support is means tested, so you will only qualify if your income is too low and you don't have lots of savings - your partners income and savings are taken into consideration if you have one.