Finances, adult children in FMH & grandchild

Hi, I have found this site a great support over the past 7 months and now need to ask advice. I won't bore you with too much detail!
My xtb after 30 years of marriage and 4 children is co-hab with woman same age as out daughter. He tells me he is renting a room BUT they and her son have gone on holiday together, shop together and enjoy family outing with her six year old son. He also bragged to my brother-in-law about his sex life and has her naked photos on his phone (20yr old son not happy prob 'cos parents shouldn't do that) All happened after our grandchild born drug dependant 10 mths ago. Now grandkids being removed. 6 yr old to live with me 10 mth old to live with her paternal family.xtb has filed for divorce. Fictional grounds except I did cut up some of his clothes! he has also put costs against me. I have accepted divorce but rejected costs and am awaiting hearing. Have agreed financial split three times then he changes his mind. First hearing in April. He refuses to pay for anything other than half mortgage. Won't pay insurances or endowment.Youngest son should have gone to uni last Oct but couldn't due to my financial probs. I earn good salary but large house, mortgage etc My questions
Do I need to prove he co-habs? Will it make a difference? I want to provide a home for my 2 sons living with me but both over 18 will that make a difference? What about grandson? If he won't provide for him should he have access - what worries me is that he obviously has more money so can spoil grandson whereas I am very stretched financially and need to continue to work fulltime to pay all costs inc childcare. No support for special guardians when family other than family allowance. Can anyone advise me please?