1) Yes, at least in the Android app, but I would recommend choosing a third setting, "reply from last number used with contact", which is not currently under the settings for the Google account that your SMS or Google Voice is associated with (as I presume the reason being that they could be on different Google accounts), but rather under the SMS options (also in Hangouts settings).
Additionally, a somewhat recent update allowed to change the number to SMS/MMS 'from', in the same way that you were able to toggle SMS vs. Hangouts: by clicking the icon to the left of the in-edit (draft) message. This is very helpful for new messages, but you may want to ensue that the Google Voice number is associated with another Google account, which may require using a SIP number (YouTube 'Google Voice without an existing phone number' or similar). If Google removed the requirement that Google Voice have a phone number to link to. This method may also require that you be aware of the multiple Google accounts toggle, hopefully made easier by notifications and unread messages counts/indicator).
Note: I had messages hang when sent from other Android devices and when I chose SMS . I am not sure if it was Google Voice and/or my carrier number that hung. This defect, likely tied to the lack of the feature above, if missing for PC/Chrome OS, would need to be fixed.
2) I believe this is really one of the highest priority features for Google to undertake for Hangouts. Verizon can sync text across devices, and this was the only reason one of my roommates used their messaging app.
3) I think you are discovering the pros and cons now. I would that Hangouts has developed pretty rapidly compared to other apps that I have had that defined to be missing critical features, so you may to stick with it.
01-07-2015 12:16 PM