Soft return is alt-cmd-return (Edit > Insert > Line Break). i.e. Same as it is in TextEdit, except that TextEdit doesn't have a keyboard shortcut. As for being able to view invisible characters - I'm afraid not. Only certain fonts support this, anyway. If Apple ever embrace this in full then so will Scrivener.
Yep, the problem is that occasionally I need multiple spaces (in technical documents). Maybe, I should change the workflow and reintroduce them after cleaning the unwanted spaces. Thanks for the tip, though.