Nil Rate Fund never established by trustees

Hello reddit, my partner recently started digging into her dad's will and we could use some clarification.

Obviously getting specialist legal advice but appointments are hard to come by this time of year and some of it may be time sensitive. We, and the estate in question are in England.

- Dad died 2016. Grant of probate issued ~3 mos after death based on his will written early 2007.

• SM gave my partner a small amount of cash (£15k) in 2016 to help build a deposit for a house and said it came from the life insurance settlement. SM led my partner to believe Dad left everything to SM but that on SM's death everything would go to my partner. We now know this isn't quite true.
• Will includes the establishment of a Nil Rate Fund. My partner and her sister (deceased, no children) are 'beneficiaries absolutely' and my partner has exceeded the age stipulated. Stepmum (SM) is beneficiary of the residual estate.
• SM is named as one of 3 lay trustees/ executors. SM is given sole power to remove or appoint trustees and there is also a paragraph about SM being allowed to promise payment to the Nil Rate Fund
• The house was worth at least £500k in 2016 (year of death) and there was ~£150k in cash assets in Dad's estate. The cash assets were apparently invested in renovating the house. Property value in 2025 is ~£975k.
• The title deed shows a restriction added a month after the will was signed "No disposition by a sole proprietor... under which capital money arises is to be registered unless authorised by an order of the court."

As far as we know, no trust was ever created. The trustees also didn't act to vary or dissolve anything in the 2 years following Dad's death.
This has all come to light because SM is trying to sell the property to free up capital for retirement. This is understandable but we think it is largely at the behest of a new partner who we distrust deeply for reasons which aren't salient here. For that reason we're also pretty concerned about what my partner can expect from her Dad's estate or not.

There will probably be an opportunity for a frank conversation with SM over the holidays but my partner doesn't know where to even start.

TL;DR? Trustees never set up the NRB. What rights do beneficiaries have?

Our big questions are:

1. Can SM undo the trust and effectively keep the entire estate herself?
2. What power does my partner have in this situation, if any? She'd prefer to stay cordial with SM but is willing to go scorched earth if necessary.
3. What are the next steps?

I have access to a copy of the will if further detail would be helpful.