1. still 400w each... so you need at least 800w @ 4ohm...

2. wont say save or not... because it all depends on how much the sub could really take... and also make sure you do your scoping on the amps to really know where does it clips and stuff... that way you can keep your setup safe...

3. yes each sub will get 550wrms... the other channel... i guess your RCA will have to use a mono RCA... best not to feed the other unused channel with audio signals with no output.

if you are not going for a serious SPL competition... using this setup for a long run...

you should use both the channel... 700wrms x 2 @ 8ohm for both subs... that way you run your amp at a safe operating region while giving both subs the right amount of juice to work properly...

the clip function... yea its used that way... be its still better if you scope your amp... its a small fee to pay to protect your investment...

and to tell you the truth... when you are running a full potential 1000wrms SPL setup... you will have to reinforce a lot of things or else you would get a lot of rattling and "self dismantling" of a lot of stuff... plus... your ride's charging system would be choking as well trying to supply those huge current... which in the end, 3 things happens:
1. sub voice burnt, due to "overjuiced"
2. amp failure due to insufficient current supply from your ride's charging system
3. ride's charging system failure due to constant overloading...

your setup... your call