Can you use the arrow key to complete an entry in point mode?

In some cases, yes, and in others no. You can use the arrow keys to select cells or the mouse in point mode. If the last entry isn't a cell in your formula, then you can use the arrow keys to complete the entry. So the following formula could be entered using the arrow keys or mouse to select cells and completing it with the arrow keys:

=A3*10

The following formula could not be:

=A3*B3