May I treat a 2D array as a contiguous 1D array?

Both lines do result in undefined behavior.

Subscripting is interpreted as pointer addition followed by an indirection, that is, a[0][1234]/p[1234] is equivalent to *(a[0] + 1234)/*(p + 1234). According to [expr.add]/4 (here I quote the newest draft, while for the time OP is proposed, you can refer to this comment, and the conclusion is the same):

If the expression P points to element x[i] of an array object x with n elements, the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element x[i+j] if 0≤i+j≤n; otherwise, the behavior is undefined.

since a[0](decayed to a pointer to a[0][0])/p points to an element of a[0] (as an array), and a[0] only has size 80, the behavior is undefined.

As Language Lawyer pointed out in the comment, the following program does not compile.

constexpr int f(const int (&a)[2][3])
{
    auto p = &a[0][0];
    return p[3];
}

int main()
{
    constexpr int a[2][3] = { 1, 2, 3, 4, 5, 6, };
    constexpr int i = f(a);
}

The compiler detected such undefined behaviors when it appears in a constant expression.