No problem on the advice, it's free
The Pioneer GM-D7500M does not have preamp outputs (RCA passthrough) so that doens't help. You could always upgrade your factory door amp to one that supports passthrough and really get those door subs hitting
Regarding option one you may lose a bit of volume in both door and trunk subs but I'd be more concerned about the noise in the signal. As you reduce the signal amplitude the noise becomes more apparent... If you have a 6V signal coming from your head unit with 0.1V of noise you might not notice the noise since it's fairly insignificant compared to the overall signal strength. On the other hand if you have a 2V signal with 0.1V of noise, the noise will be much more significant since it is now 5% of your total signal strength. Don't get me wrong... I'm not trying to scare you off from this approach. It may work perfectly for you particularly since you are driving subwoofers which are less sensitive to high frequency noise than a mid range speaker or a tweeter would be.
Regarding using the rear speaker outputs w/ a low pass filter... idk... your head unit may turn off the wiring harness outputs if it detects an RCA output. Then again it may not turn them off. No clue.
Again another options I've heard of people using is to splice a "line out converter" directly into their rear speaker wires (at the rear speakers in the trunk). This will draw some power from rear speaker IIRC, but how much power is drawn probably depends on the quality of the converter. The Line out converter will convert the signal in the speaker wire to an RCA connection and I believe you can get some that have a built in low pass filter. That way you could splice right into the rear speaker wire, hook up you amp, and run it that way... Again, I've never done this but have known people who did with decent results.
2005 Windveil Blue GT (MT)
93H Bama tune, JLT Series 3 CAI, FRPP CMDPs, Pype bomb super system, 3.73 gears, Steeda shift bushings, Saleen short shift block
Steeda ultralite springs, Tokiko D-specs, J&M Extreme joint UCA/LCAs, PHB, C/C plates, bumpsteer kit, Steeda Q1S wheels, 2011 GT brakes, 275/40R18 Sumi HTR Ziii tires