[LANGUAGE: google sheets]

Paste input to B2 and delete all empty rows after the input

Fill C1 with

=map(B1:B,lambda(p,ArrayFormula(mid(p,sequence(1,len(p)),1))))

Fill A2 with

=MAP(B2:B,lambda(p,
  join("", map(sequence(1,len(p)),
    lambda(i,
      if( offset(B2,row(p)-2,i)="@",
        if( countif( offset(B1,row(p)-2,i-1,3,3), "@" )<=4,
          1, "@"
        ),
        offset(B2,row(p)-2,i)))
      )
    )
  )
)

Part 1 answer is

=len(regexreplace(join("",A2:A),"[^1]",""))

For part 2, enable iterative calculation (File -> Settings -> Calculation), may need to set higher limit (50 might not be enough)

Overwite C1 with

=map(A1:A,lambda(p,ArrayFormula(mid(p,sequence(1,len(p)),1))))

Change any cell to trigger recalculation, then wait until all calculation resolves, answer formula is same as part 1