The sore thumb from loading the mag can be remedied with a clever plastic sleeve that goes over the loading button, it is called a "thumb saver" and makes loading a breeze.
Before the "thumb saver" came along I just used the universal cowboy tool: a bandana. A couple of wraps around your left thumb does the trick, the cloth snags the little button and you can load a dozen of them in a row without the slightest problem.
If you don't have a bandana or thumb saver, a regular old plastic bottle cap off of a 20 ounce soda or 2 liter bottle will work great as well.
As for the disassembly / reassembly, it is a piece of cake. A small piece of cord or wire will let you grab the mainspring latch and lever it out without marring the gun's finish (I've seen people dig at it with a screwdriver before and gouge it up good

). I use a bandana, works fine lasts a long time and I always have one in my pocket anyway :-laf :-laf.
On reassembly, make sure you tilt the gun's barrel up a bit so that the hammer strut falls into the right position - you can see it easily, it is a little "flap" that has to be aligned correctly so the bolt will open/close. If you don't tilt the barrel up when you reassemble, invariably the fiendish little gizmo will flop out of position and when you close the mainspring the bolt will not open or close. Of course, I have only heard of this happening to OTHER people, there's no way that I would have nearly had a heart attack trying to reassemble a used Mk II that I bought without an instruction booklet many years ago, NOSIRREE :-laf :-laf :-laf
Takes longer to describe than to do - literally. There are some videos on Youtube that show how to do it. Once you have done it a few times, you will really appreciate the genius of Bill Ruger's design.
Have fun, they are GREAT pistols!