Is it safe to play a scene without a safe word?
No. Whenever basic/main rules are reduced, then boundaries get confused and knock-on effects mean that injuries are more likely.
I speak from experience.. I tried this for the first time 2 weeks ago. I thought the usual rules weren't needed as it was a spur-of-the-moment, quick, simple whipping on the butt.. result.. an unplanned blood nose when the sub jumped.
But how would a safeword have prevented this from happening? Accidents happen...all the time...often less so during SM scenes, as people are aware of the dangers. You don't need a safeword in this situation - the bottom couldn't have known she was going to 'jump', and you certainly don't need a safeword after the event, who wants to play with a bloody nose? (Actually...I know some people, but let's not go there...

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I have played, as both a top and a bottom, without safewords, I have never injured anybody, nor have I been injured. I have written at length about safewords on here before, and would recommend them for new players, new partners and when doing psychological DS. My personal preference is not to use them in SM scenes, but to use a 'Time Out' to inform of something wrong (eg. hands going numb, cramp in leg etc). But a safeword that means 'stop play, I'm having a hard time'? Not for me...if I'm bottoming, I expect someone to be able to read my reactions and make some effort to understand me (hence the proviso of new partners)...if I'm topping, then I expect to do the same back.
Obviously, if someone wants to use a safeword, I wouldn't say no...and I would never say that this type of play is for everyone. It's a personal opinion/choice.
Be people actually use three safe words (slow down/speed up, less/more) or would you expect a dom to be competent enough to read their sub?
In my opinion, even with an experienced Dom it pays to have 3 words - used for Stop, Time-out, and Slow down (although I only contractually agree to act on the first two as safe words). I guess some could use a fourth - speed up, although I don't always give this option and it has never been said to Me.
I enjoy timing it so that I know exactly when a sub/bottom is going to use a safe word and I think it adds to the enjoyment of the scene when I watch someone struggle with their own limits against using a safe word. I never play without safe words, it is part of being a competent Dom.
OK...here I really take issue with you - if you hand over control to a top, then you do just that: hand over control. If I was topping someone and they used a safeword to say 'slow down', then I wouldn't like it one little bit. It's my job as the top to decide pace etc...I do this by reading body language, breathing etc...not by being told what to do. I'm a competent top (I prefer this term to dom)...and the people I play with trust me to know what I am doing. If they didn't, and wanted a whole slew of safewords to cover every eventuality, then I wouldn't play with them.
SM isn't always supposed to be safe...it isn't always supposed to be controlled...it's supposed to be hot and sexy and one wild ride. Obviously, there are basic levels of safety to be adhered to (I don't have a back garden to dig graves in, so I tend to be careful not to kill anyone!), but it is possible to let the 'safety' aspects outweigh the fun ones...not a good move in my book.
kitty