I'm going to try and take this one area of the car (or one section of the amp) at a time to keep it from getting confused.
2-Skar t1t tweeters,skar 3.5” and 6x9“ on fronts side to side
When you say t1t tweeters are you talking about the ones like these: Skar TX-T
Skar 3.5" would be their coaxial if I'm correct: Skar TX35
Then the 6x9: Skar TX69
Correct me if I'm wrong on any of those.
So if you have those 3 units wired in parallel then from 1.6kHz and up you have 3 x 4 ohm drivers in parallel = 1.3 ohms but that's all in the tweeters and they use so little power that it wouldn't be a huge problem. Still getting rid of one set of tweeters or the 3.5" set should make this front channel setup doable in parallel.
top of the box is 2x DS18 sealed back 8” full range,Skar 3.5” mid and DS18 higher quality tweeters. This is connected to the rear deck parallel.
back window is a 2“ tweeter and a 6x9”
rear channel + box is 10 speakers.
This one you lost me - we don't really need to worry about the subs as they have their own amps.
DS18 sealed back 8 + Skar 3.5 + DS18 HQ Tweeters + 2" Back window Tweeter + 6x9 are all on the rear channel of the amplifier?
The DS18 Sealed Back 8" are only available in 8 ohm IIRC so that's good from 250Hz - 10kHz
The same Skar 3.5 as above is 4 ohms from 145Hz - 20kHz
The DS18 HQ Tweeter I'm going to assume is one of their 3" Bullet Tweeters at 4 ohms from 2kHz - 20kHz
Same for the 2" back window tweeter = 4 ohms from 2kHz - 20kHz
And the Same Skar 6x9 from above = 4 ohms from 40Hz - 25kHz
If all that is correct then the worst area is from 2kHz - 10kHz where all this is in parallel so you would have 0.88 ohms
Then from 250Hz - 2kHz where you have around 1.6 ohms
Then from 145Hz - 250Hz where you are at 2 ohms
and then 40-145Hz is at 4 ohms
Getting rid of the 3.5 and the 2" tweeter would help tremendously. Ideally you would have the 8" sealed back and the HQ Tweeter on the rear channel and then put the 6x9s on their own amp. So maybe get rid of 2 sets of the little tweeters and the 2 sets of 3.5s and see then see if your thermals are more manageable - if it still gets hot then disconnect the 6x9s and try again.
The DSP is just going to be a glorified EQ in your situation, Time Alignment might help a little but a DSP is really meant to have one channel for each driver in your system. Between using a combination of coax and separates and the sheer number of speakers you have the advantages of DSP are going to get lost in the mix. Best to get it ironed out as much as you can without the DSP 1st and then if you still need more adjustment consider the DSP.
When you measure with the multimeter are you measuring with all the speakers connected or each speaker individually?
Are you using any kind of xover other than what came with each driver? Or do all the drivers connected to each channel receive a full range signal?