Slats/spindles are usually held in place with mortise and tenons. The
shoulders of the tenons, with the tenons sitting in a mortise a little
deeper than the length of the tenon, is what normally governs whether the
two rails are parallel.

You are obviously not doing this.

Question #2: Do you think bscuits will be a strong of enough joint for
the
slats? There is no up and down forces pulling on the joint so I was
thinking biscuits would work.

I've never heard of using biscuits to hold slats/spindles in place, but,
and
since strength is not an issue, and if the slats are thick and wide
enough,
why not?

The slats are 5/8" thick X 5 7/8" wide.

I might switch my plans back to M&T's. I thought that 18 biscuit joints
would be easier to do than 18 M&T's. But if I am sacrificing a lot of
strength than maybe I should suck it up and do the M&T's.



I often don't even glue slats/spindles into place when using M&T joints.

There is another way to do this ... with the slats housed in grooves that
run the length of both rails, with spacers the same thickness as the depth
of the groove between the slats as spacers.

I've never liked to do that and you still have to get the slat length
perfect to keep the rails parallel.

Good food for thought! Thank you.