[angott]

> most frustratingly I can’t send an SMS to any contact it believes to be on iMessage

You can long press on a sent iMessage blue bubble to reveal a 'Send as Text Message' menu item. It's quite hidden but it's there.

[lxgr]

Only if the message didn’t go out to Apple’s servers. That doesn’t help me when I know the recipient won’t be able to receive it (e.g. because they are on roaming or out of mobile data).

[speg]

If you know you want to send an SMS you can tap it right after you press send, there’s a small grace period. I just did it.