Im suprised also about the 3+ lbs more. I would take a guess that the tires account for part of that, the tubes are heavier for sure, the rims?
The 36 spoke stock rims on my wifes 2017 Troll are a fair bit heavier than the 32 spoked on my bike, but perhaps you transfered the wheels over too?

My C17 has softened up from when I got it used, but practically new. I wasn't keen on it compared to the leather seats that just worked so well for me, as I could clearly feel my sit bones more on the C17 at first. I ended up sticking it in a shelf for along time, but finally put it on my indoor bike during the winter on a trainer, and it started to feel not so bad (hence my theory it softened a bit, more flexible) and finally last summer I put in on my "faster" bike, and it was ok.
I am fairly sure that being on the lighter faster bike, I would stand sometimes and such, so not always seated. Maybe I got used to it a bit and toughened up, but I think its fair to say still than my leather B17s are simply more comfortable for long trips, as I really have no issues with discomfort, better than any seats from the past anyway, so I stick with the "if it aint broken, dont fix it" thing---but I am aware that if on a trip with lots of rain or whatever, the C series are certainly easier in that you never have to worry about them out in the rain.

have fun on it. I've said in the past, one day I will redo my troll with jones bars that I have in the garage, and go fatter tires like you, and do a predominantly off road trip.

ps, I just saw that supposedly ET's come in a 26x46mm version , bottom of page
https://surlybikes.com/parts/extraterrestrial