You're welcome. I've tried to answer your questions in turn below:

I assume by your answer that Fraser Double Rifles do have a "Through-bolt" system.
A: Given the pictures I've seen of your rifle, I would "assume" yes, but I've certainly seen much stranger things in the realm of vintage gun.

1. What is the "hand pin"? Is this the screw under the trigger guard that comes up from the bottom and goes into the end of the top tang? Or is it something else?
A: Yes

2. Are the through bolts typically centered in the stock, between the recoil pad attaching screws?
A: Not necessarily. However, this is Fraser and it would not surprise me in the least if the thing was! The reason being, is that with a such a svelte stock you are dealing with many curves, slopes, transitions, casts, and multiple planes - no less intriguing than the lovely legs that grace certain members of the opposite sex!

3. How far from the butt end of the stock (beneath the recoil pad) is the end of the through bolt? Is it near the end, or far into the stock?
A: It all depends on the stock's length of pull. I think I recall it being somewhere around 6"-8" from the butt of the stock. Roughly halfway through the stock.

4. Does the through bolt have a turn-screw slot to loosen and tighten it, or a square head?
A: I recall it having a slotted-head screw

I've checked my Fraser picture portfolio and don't have a picture of the internals of his boxlock. This shall soon be remedied! I believe nitroexpress member MikeRowe has a photo. Maybe we can get him to graciously share? He shares my clinical infatuation with Scottish makers.

A few additional thoughts:
I certainly applaud your curiousness and interest in digging in to it. Many would tell you to abandon it. As long as you can properly wield a turn screw and a few other tools, and do no harm, then I say go for it. As for the triggers, I'd leave them as is (and I'm capable of making the adjustments

), but that is just me. The aging you mention is a bit of a challenge, and it's good that you noticed it. With the right skill, some eye of newt, and hair of bat they can sometimes be aged back evenly, providing no proof that someone has been into it. I've accomplished it before, but I certainly wouldn't guarantee I could repeat my success. When I restore guns I do not wish for them to look new, even when I put on a new finish! Having said that, there are other options. I once saw a Mannlicher Schoenhauer fitted with a Silvers pad that had a graceful ellipse cut out of the central portion of the pad. This cutout provided access to the Mannlicher hinged plate. Opening the hinge provided access to the segmented cleaning rod and oiler - very clever! It is possible that this could be done in similar fashion to provide access to the through-bolt of your Fraser, though I'd encourage it be done with the utmost care to stay stringent to the graceful lines and workmanship that Fraser is so well known for.