Don't put the speakers anywhere that causes them to be obscured. If they need to be in front of the screen, so be it. I don't know that 19 degrees is too narrow a separation, but I expect two things - First, that you should be able to tweak that position enough without changing where and how you wire the room, and second, that you should leave as much room for future larger speakers as possible. Hopefully someone else can comment on the angle considerations.
In response to your other questions more directly:
I don't know if you'll be able to use the sub at all, but I'm not too optimistic that you can use that sub without the rest of the system.
The primary benefit of multiple subs in any home-sized room (an acoustically small room) is minimizing the modal coupling of the LF drivers to the natural modal response of the room (or perhaps more properly, to drive modal waves from multiple positions, to avoid strong coupling - not sure what the best terms are here) . This has two major outcomes - first (and of lesser interest to you) is minimizing the seat-to-seat variation in low-frequency response across the listening area, and second - the ability to strategically locate and equalize the subs/signals to avoid the nulls and peaks associated with modal ringing. Additionally, you can get extra headroom in the system so that global EQ can be more effective. As a side note, and along the lines of what djkest said, the low frequency performance of your small Bose cubes will be very poor. Hopefully the VCS-10 is better - it should be I think, since it has a reflex design it's probably carefully engineered to reproduce normal dialog, which can include frequencies a good bit lower than the 250Hz that djkest sites. This is of mention because many subwoofers are not terribly good to that high a frequency. You might look into something like HSU Research
's Subwoofers and Mid-Bass Modules - they could be a high quality and reasonably affordable way for you to bridge your current speakers into a more traditional setup with at least mains with wider usable frequency response.
In an ideal setup, the front soundstage (L/C/R) is ear/eye height, and positioned behind the screen (where appropriate based on both the separation angles for primary listeners and the screen material, obviously). Some compromise may be made to accommodate the various heights of listeners on separate rows.
The height of the surround loudspeakers is generally greater. The reasons for this include avoiding the sound shadow that one listener can cast on another, and minimizing the difference in distance between the speaker and each of the listeners. (Do you see the triangle and how raising the speaker makes the distances more similar?) The distances should be similar to allow for a setting delay and level that is acceptable for both seats. Two feet is a common recommendation and should be appropriate for most small rooms. If it's not possible to aimthe speaker down into the listening area, care should be taken to keep the listeners within the normal on-axis (vertical) listening area for the loudspeaker. I have no idea what the soundcubes' off-axis response looks like, but I would guess that for most surround loudspeakers 15 degrees below the normal axis is a safe maximum (that's a guess).
I personally think more subs is a good upgrade for any common setup, but the proof will be in the pudding. Wire is cheap - why not run some wires to a whole bunch of places? (I'll probably wire for 6 or 8 potential locations in my 12x22 theater, and use two for sure, hopefully 4.)