Exposure is the right thing to do.
Why lie to your daughter for your husband and his enabling mother?

I had an extensive talk with her three days ago. She didn't seem hurt, after all she is 5,5 and she has heard part of it before. She already has some misconceptions like when she said " I get it, because she is blonde and beautifyl dad chose her", so I tried to explain what marriage and love is to her (although he has admitted that I am more beautiful than her, you know us women care about those things). So now it is his turn to try and deny it to her, because he insists that although he couldn't do NC even for 2 days, this is NOT the reason we failed.

Now you need to expose to everyone else.
Read the Exposure 101 link and prepare an Exposure letter.
Post it here first for review

What is the status of your divorce case?

It is a no fault "mutual" D. Nothing has been signed yet. We have the same lawyer.Once signed its gonna take only one month to finalize . He is not saying to anyone that we are divorced while I do. News are traveling fast of course in our small town. About OW parents: she is a serial cheater and her H and they know it .They (the parents) just don't know about the divorce she caused. Is there any point for me talking to them now? And am I wrong to tell everyone who asks that I am getting a D? He has not make any move towards me he is just being polite. Since I am in plan B I have no idea about his intentions.

Yes. Be honest and expose the affair to all of OW family and friends.
Read te Exposure 101 thread.
Some affairs have been killed by the mother calling her sons affair partner and telling her she will never be welcome into their family.

Ensure all relatives and married friends on Facebook (Ow and husband) receive exposure letter.

Read the thread and post your letter here for review.

As for your divorce case, what is the custody agreement?

50:50

Do you think that it is not the same if BH tells his ILs? Maybe they won't believe him?

You don't know what he did or didn't do
I would expose it to them
In my case I exposed it to all of OM family.

As for custody why are you or seeking full custody?
Do you want OW raising your daughter?

Because he will probably spin the story or flat out lie. Please tell them yourself and OW's family.

Please use the templates in here. Exposure 101

No, what I meant was that the OW husband can tell our story to his in laws (he told me that he may do that ). As for custody you make me nervous now. I don't want her near my daughter but shouldn't my daughter have visitations with her dad? I have also thought of filing for protection from her on the grounds of profound insult or smth like that because she (or my WH I don't know) said to her husband that I have been hospitalized for mental issues! Actually when I heard that was the final straw and the next day I filed for D.
As for exposure I said in my first post that I have done it.

FNM

Just my .02 cents from my experience.

I waited a long time to expose my WW. When I finally did I sent letters to all I thought could influence her.

Come to find out one sister and brother out of 6 siblings, and "Baldos" (POSOM) parents knew nothing of the A even though they are a close family.

Between those four they found my WW unworthy of their son and brother almost to the point of beating her up physically. They wanted nothing to do with her.

My FIL severely disliked Baldo. Had met him at social events and the guy reminded him of a weasel. Usually my FIL des not talk about likes or dislikes with people but he had strong feelings about Baldo.

Wow I feel so bad that his parents/siblings when they knew rejected her and her own family knew and rejected him. Makes it hard to re-introduce the OP 6 months down the road after the smoke has cleared from the D.

Just goes to show you never know who can help you or try to influence the wayward. I exposed about a year before I filed. If anything neither are welcome at each others gatherings or get togethers.

Just breaks my heart when that happens.(FULL out and out SARCASM)

nesre

Okay.
Regarding custody, what is the honest truth about any moral defects in character of OW; does she have any addictions or criminal record?
What about your husband?
Yourself?

Regarding the claim that you have mental illness, wayward spouses make all sorts of claims to their affair partners. My wayward wife tol me her affair partners wife was a lesbian and denying this poor man sex, and her affair partners wife told me that she heard I had autism. Just disregard it and focus on more important issues

She is a serial cheater and a professional mistress (has only dated married or engaged men in her life). I have info that she does drugs (joints) with my H. Maybe unstable personality. But no proof for any of the above other than her husbands word.

If she is a drug addict or even "recreational" drug user you don't want her or your husband to have custody.
I encourage you to contact an attorney (not the one you are currently seeing jointly).
Tell the atty about the drug use. Tell the Atty you want a GAL (Guardian ad litem) appointed to represent your daughter. You also want the court to order supervised visitation with required drug testing.

DO NOT TELL YOUR HUSBAND this.
DO NOT LEAVE the marital home.

Do you use any illegal drugs?

As for proof the answer is court ordered drug testing and witness statements

No I don't , apart from the occasional sleeping pill.