Over many years living in Calif., I backpacked, hiked, camped, and hunted and fished all over much of the Sierra Nevada range, so know a fair amount about much of it.
(BTW, folks, the correct name is "SIERRA," not "Sierras," or "Sierra's." In Spanish, the word "Sierra," is plural, meaning a collection of mountains within a rocky mountain range. Therefore, the "Sierra Nevada," or, "the snowcovered mountains range.")
That little bit of trivia out of the way, I've not seen this in any of the responses, but as far as "trapping and snaring, and fishing," you'd better have a Calif. Hunting license and fishing license with you at all times! Go to the Calif. Game & Fish Dept., and not only buy your two F&G licenses, but pick up a copy of the hunting and fishing regs and study them so you know what is legal and what is not.
Don't depend on those high mountain lakes and streams for trout. Sometimes yes; sometimes no. Plan on "no."
As far as carrying a gun while backpacking, the Hunting license insures your legality, but..... there are different laws about carrying concealed and non-concealed out in the boonies. Both the Calif. Penal Code 120XX defines where you can carry concealed and not. Those Calif. P.C. laws at times, conflict with the Calif. Fish & Game laws. KNOW THE DIFFERENCE.
As said by others, you can not LEGALLY carry a firearm in a National Park or Calif. State Park. Always know where the boundaries are up in the Sierra.
Another thing I haven't yet seen mentioned about backpacking in the Sierra, but is of EXTREME IMPORTANCE is carrying and using at all times, a top rated water filter!! Do not drink the water from streams or lakes in the Sierra! Most are contaminated with Giardia, or "Beaver Fever." I prefer the Katadyn filter.
Anytime you're above 6,000 feet, you need to drink a copious amount of water to combat altitude sickness, which comes mainly from dehydration at high altitude. So take that Katadyn filter and USE IT!!
.45 ACP? .357 Mag.? In my opinion, as you already have a .45 ACP and are comfortable and familiar with it, I'd pack it and a couple of extra magazines. I've carried my Colt's Lightweight Commander many a mile in the boonies and never worried about the caliber. Also have packed my old Ruger Security Six SS 4" .357 many times. Either is just fine for what you're going to do.
Rather than investing in a new pistol, ammo, holster, and practice just for a backpacking trip, I advise you put some of that money into that Katadyn Water Filter of which I spoke.
Others here have commented on cutting down your pack weight, type of food to take, clothing, etc. I won't touch on that, other than to say that many years ago when I was in the Army (no combat), in addition to my issued M1 Garand rifle, I carried about 65 pounds when we'd head out for field training, etc. I was then 20 years old, 6'3" tall, weighed 178 pounds and was in very good condition. Even so, that weight was plenty for me, and we did not even go into the altitudes which you listed.
You mentioned renting mules to pack some of your goods. Hmmmm.
I don't know how experienced you are with four footed beasts of burden, but I've done a fair amount of horse and mule packing both in the Sierra and several of the Rocky Mountain states. I can assure you, you don't know what
"fun" is, until you've tried to untangle a mule wreck on those very narrow, dangerous, steep, switchbacks in the high Sierra!!
Anyway, these are my suggestions. Take them for whatever use they might be, if any.
Have fun on your trip.
L.W.