I read at UESP that effects are cast from the bottom up in custom spells. To me it looks like they are cast from the top down.

They are. The wiki is wrong.

Yes. They are cast from the top down. The wiki is wrong on this. There was a thread on this a few weeks ago, where I posted some detailed examples of why. Might be difficult to find it, but if I do I'll post a link.

But it is cast from the top down. Other rules: weaknesses in the same spell do not affect damage in that spell. And a second cast of the same effect in the same spell replaces the first.

EDIT: Here is the http://www.gamesas.com/index.php?/topic/1173643-enchanted-weapon-effects-applied-in-reverse-order/page__p__17327707#entry17327707 .

That is good to know about the weaknesses.

I discovered that you can't use a custom spell to train more than one skill.

Is it possible to sell spells? I need to recreate many of my custom spells.

Yes multi effect spells will only train one skill.

You cannot sell or delete spells in game. You can delete them with the console, or with mods, but that is on a PC only.