OK, here goes. Let $p+1-k=\ell$. If you have any from $\ell$ directions not from your pencils, then the sums along the lines in that direction should not depend on the line and (since the whole sum over $\mathbb F_p^2$ is fixed), not on the direction either. This is the only property we'll use below. Let us call this line sum $S$. You want to find $2\ell$ non-zero values.
Case 1: $S=0$
Since the functions you add are orthogonal modulo constants, you cannot have an identically constant function in the end, so there is some nonzero number somewhere. Assume it is positive. Then on each of $\ell$ lines through the corresponding point there should be a negative number. Thus the negative numbers are at least $\ell$. Take one of them and run the same argument to conclude that there are at least $\ell$ positive numbers too.
Case 2: $S\ne 0$:
This is where the real fight is. Assume for definiteness that $S>0$.
Note that if for some present value $y$ the value $S-y$ is not present, then each of $\ell$ lines through $y$ contains at least $2$ other non-zero numbers and we get $2\ell$ immediately.
Assume that there are negative numbers. Let $x<0$ be the least of them. Then $S-x$ is present and is the largest value in the game.
Let $a$ be the number of $x$, $b$ the number of $S-x$ and $c$ the number of everything else. Now take any line through $x$ that does not contain $S-x$. There are at least $\ell-b$ such lines and each of them contains at least $2$ numbers not in $\{x,S-x\}$ (here is where I use that $x$ is the least and $S-x$ the largest value and that $x<0$, so adding $x$'s to the line only makes it harder to recover). Thus $c\ge 2(\ell-b)$. Arguing in the same way about the lines through $S-x$ not containing $x$, we get $c\ge 2(\ell-a)$. Averaging, we get $c\ge 2\ell-a-b$ and we are done again.
Thus every non-zero value is positive. In particular, any line through $S$ cannot contain any non-zero value.
Let now $u$ be the number of $S$ and $v$ the number of everything else.
Since every line in our $\ell$ directions passing through something other than $0$ or $S$ should contain at least $2$ non-zero values, we have $u+\frac v2\ge p$.
Now again 2 cases
Case 2a: The points with the value $S$ do not lie on a single line. Then we have the inequality $\frac{u+3}2+\ell\le p+1$ (because all directions determined by those points are not among our $\ell$ directions). Thus
$u\le 2p-2\ell-1$, $v\ge 2(p-u)$, so $u+v\ge 2p-u\ge 2\ell+1$.
Case 2b: The $u$ points are on the same line. If $u=p$, then there cannot be anything else anywhere, so this configuration is the one with a single pencil, which we excluded. Otherwise $u<p$, so $u+v>p$ and we have some point with non-zero value not on that line. That point determines $u$ directions to the points with value $S$ that cannot be our $\ell$ directions. Thus
$u+\ell\le p+1$, $v+u\ge 2p-u\ge p-1+\ell\ge 2\ell$.
The only non-trivial case is $2a$. But it has to be so because the property I mentioned in the beginning characterizes the functions that are sums of $\le p+1-\ell$ pencils of your type, so if we could find $p$ points not on one line with $<p/2$ directions, that would be a counterexample to your statement. That's why I asked about it.
