validating datamodel with biber complains about items used with crossref mechanism

These messages are perfectly sensible according to the standard data model: @books don't have a booktitle but should have an author.

Things get much, much better when you make HPSGHandbook an @collection. In that case you won't get a warning about a missing author (because @collection is not supposed to have one) and you won't need the booktitle, because the title will be automatically inherited by a child @incollection as booktitle.

documentclass{article}

begin{filecontents}{jobname.bib}
@incollection{AC2020a,
  author   = {Anne Abeillé and Rui P. Chaves},
  title    = {Coordination},
  crossref = {HPSGHandbook},
}
@collection{HPSGHandbook,
  editor    = {Stefan Müller and Anne Abeillé and Borsley, Robert D. and Jean-Pierre Koenig},
  title     = {Head-Driven Phrase Structure Grammar: The Handbook},
  year      = {2020},
  address   = {Berlin},
  publisher = {Language Science Press},
}
end{filecontents}

usepackage[
  backend=biber,
]{biblatex}
addbibresource{jobname.bib}

begin{document}
autocite{AC2020a,HPSGHandbook}

printbibliography
end{document}

---

I really don't see the point in changing the data model in a way to allow the bad input to pass. The whole point of data model validation is to pick up issues in your file that you might not see when you just look at the output.

If you insist, here is an MWE that includes a changed data model that adds booktitle to the known fields for @book and drops the requirement to have an author. It's quite long, because the data model constraints have to be restated (there is no interface to remove a single mandatory-field constraint, see also the comments below my answer to How to cite report-like documents that do not have/need an institution?).

documentclass{article}

begin{filecontents}{jobname.bib}
@incollection{AC2020a,
  author   = {Anne Abeillé and Rui P. Chaves},
  title    = {Coordination},
  crossref = {HPSGHandbook},
}
@book{HPSGHandbook,
  editor    = {Stefan Müller and Anne Abeillé and Borsley, Robert D. and Jean-Pierre Koenig},
 title      = {Head-Driven Phrase Structure Grammar: The Handbook},
  booktitle = {Head-Driven Phrase Structure Grammar: The Handbook},
  year      = {2020},
  address   = {Berlin},
  publisher = {Language Sciene Press},
}
end{filecontents}
begin{filecontents}{bookparent.dbx}
DeclareDatamodelEntryfields[book]{
  booktitle}
  
ResetDatamodelConstraints

DeclareDatamodelConstraints[
  article,
  book,
  inbook,
  bookinbook,
  suppbook,
  booklet,
  collection,
  incollection,
  suppcollection,
  manual,
  misc,
  mvbook,
  mvcollection,
  online,
  patent,
  periodical,
  suppperiodical,
  proceedings,
  inproceedings,
  reference,
  inreference,
  report,
  set,
  thesis,
  unpublished]{
    constraint[type=mandatory]{
      constraintfieldsxor{
        constraintfield{date}
        constraintfield{year}
      }
    }
  }

DeclareDatamodelConstraints[set]{
  constraint[type=mandatory]{
    constraintfield{entryset}
  }
}

DeclareDatamodelConstraints[article]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{journaltitle}
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[book,mvbook,mvcollection,mvreference]{
  constraint[type=mandatory]{
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[inbook,bookinbook,suppbook]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{title}
    constraintfield{booktitle}
  }
}

DeclareDatamodelConstraints[booklet]{
  constraint[type=mandatory]{
    constraintfieldsor{
      constraintfield{author}
      constraintfield{editor}
    }
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[collection,reference]{
  constraint[type=mandatory]{
    constraintfield{editor}
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[incollection,suppcollection,inreference]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{editor}
    constraintfield{title}
    constraintfield{booktitle}
  }
}

DeclareDatamodelConstraints[dataset]{
  constraint[type=mandatory]{
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[manual]{
  constraint[type=mandatory]{
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[misc,software]{
  constraint[type=mandatory]{
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[online]{
  constraint[type=mandatory]{
    constraintfield{title}
    constraintfieldsor{
      constraintfield{url}
      constraintfield{doi}
      constraintfield{eprint}
    }
  }
}

DeclareDatamodelConstraints[patent]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{title}
    constraintfield{number}
  }
}

DeclareDatamodelConstraints[periodical]{
  constraint[type=mandatory]{
    constraintfield{editor}
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[proceedings,mvproceedings]{
  constraint[type=mandatory]{
    constraintfield{title}
  }
}

DeclareDatamodelConstraints[inproceedings]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{title}
    constraintfield{booktitle}
  }
}

DeclareDatamodelConstraints[report]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{title}
    constraintfield{type}
    constraintfield{institution}
  }
}

DeclareDatamodelConstraints[thesis]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{title}
    constraintfield{type}
    constraintfield{institution}
  }
}

DeclareDatamodelConstraints[unpublished]{
  constraint[type=mandatory]{
    constraintfield{author}
    constraintfield{title}
  }
}

DeclareDatamodelConstraints{
  constraint[type=data, datatype=isbn]{
    constraintfield{isbn}
  }
  constraint[type=data, datatype=issn]{
    constraintfield{issn}
  }
  constraint[type=data, datatype=ismn]{
    constraintfield{ismn}
  }
  constraint[type=data, datatype=pattern, pattern=regexp{(?:sf|sm|sn|pf|pm|pn|pp)}]{
    constraintfield{gender}
  }
}
end{filecontents}

usepackage[
  backend=biber,
  datamodel=bookparent,
]{biblatex}
addbibresource{jobname.bib}

begin{document}
autocite{AC2020a,HPSGHandbook}

printbibliography
end{document}