Of course the actual line in question is infinite in length so there is no mathematical reason to stop at any particular endpoints. So I don't think it would be wrong to have it extend a bit below the baseline (as long as the top endpoint also crept down a bit so that the length matched the parallel side. It seems simplest though to just end at the baseline.
But if the line fails to cross the grid lines at certain precise locations, that would be incorrect.
As for the circle, it is required that it pass through the
centre of each O so it can't be tangent. If yours is tangent, it doesn't go through the centres.