[SJAB]

fantasy always needs a prologue

Never heard this at all.

Penguin now want dialogue in its own paragraphs,

I thought this was always the case when writing???

don’t put commas before conjunctions joining short sentences,

Huh???

one space after a full stop (period), not two...

This stems, I believe, from the fact that typists on ye olde typewriter were taught to put two spaces. (least I was during my stint in a typing pool.) Now folks tend to use one. I don't think any editor/agent is going to sit and count how many spaces there are in your manuscript. He/she is looking for a cracking story, with great characters.

As long, as John said, you have your work in a readable font, double spaced, clean and clearly presented, it will at least be looked at.