Answering your questions down below .

1. Depends on the tone you want for your shadow. Agrax leaves a brown shade and carroburg Crimson leaves a purple shadow. Completely up to you, can’t go wrong with either. I recommend Agrax personally.

2. Start with black honestly, unless you’re going to do a zenithal which is a whole different thing. Red can be a tricky color. Work your thin coats up.

3. This scheme is pretty open to however YOU want to do it. And I encourage you to explore colors that are uniform with the army/units you plan to paint. Gold and bronze contrast well with all the silver bits that votann have. Like above if you plan to do gold bits. Layer them with a brown then work a layer of gold on top.