I'm not an expert by any means but I was in the same spot as you 4 months ago and since then I've breech seated over 1000 rds experimenting. So I'll share what I've learned.

Firstly have you slugged the throat and leade area? You'll need to do this to get a good understanding of what your going to need for a bullet. From what I understand Pedersoli's have a very unique throat area.

Throats are an integral part of breech seating. From what I understand the ideal throat will have a slight taper and be up to .001 larger than groove. I made a reamer to rethroat mine as such.

To your specific ?'s
1) there is no hard fast rule as to how far the bullet is seated. At the very minimum it needs to seat in far enough that no light will shine around the edge. The depth the bullet seats can be varied from still partially in the case to up to 1/2" forward of the charge. I've tried everything in between and it seems fine....It's a little scary the first time you know the bullet is that far ahead of the charge. There are some loose guidelines I've seen. 1/16" ahead of the casemouth is one and another is some the leade imprints the first band. To be honest it comes down to doing the work shooting groups.
2) I first made a plugged case as my rifle has some camming action on closing and then I got more refined and made a palm seater that shoves the bullet in with a brisk palm. There are some really nice seaters that fulcrum off specially cut lugs or convenient points on the breech.
3)Front stuffing is it's own specific discipline. Normally one would use a false muzzle to start the bullet. Without this device it is definitely not easier.
4)I think I mostly covered this in #1 other than I wouldn't bother with Trail Boss. Personally I only use BP but if I wanted to play around I would go straight to smokeless rather than a specialist powder thats really just a high volume smokeless type powder.
5)I think I covered this is #1 as well. Yes you can use a partially full case as long as...

@Seanmp - thks for the info. much appreciated. I am by no means a target shooter and looking for the easiest way to load, be accurate and easy on the recoil, I'm initially trying to stay around 1000-1100 fps with the 300gr for recoil reason. I've got arthritis and want to start off a little low. You put aside my "safety" concerns. Thank you. I will most likely start off with smokeless until I get some experience. I actually did seat one of my bullets (that's how I got my OAL). I pushed the bullet in until I couldn't see any light looking down the muzzle. I was going to make a plugged case to push the bullet a little farther into the L/G but keep the base inside the case. I've read other places where cotton balls and toilet paper have been used to fill the case to hold the powder. A lot of options; which is good. BTW - do you know if you can use wads,floral foam, TP, cotton in smokeless applications?

The first thing you need to do is at least slug your barrel. A cast of the throat would be better, because being Italian the throat maybe big or different in some way. BTW, what is L/G?

If you don't do a chamber cast, you'll need a bullet with at least the base band .002 larger than the groove dia.

Please give up the idea of leaving part of the bullet in the case. Because of the resistance of the partially seated bullet, when the powder gas hits the base of the bullet, it will deform it and then the barrel will reform it in what ever way it can.

If you can at all afford it buy a breech seat for your rifle.

There are 2 ways to set the bullet depth. The one I've been using is called the Russ Weber method. You need a case with the head drilled out, larger than the bullet. You then put the bullet in BASE first, until it stops. You then measure the distance the rod in in the case. Take the case and bullet out, put the rod back in and the bullet on top of that. The amount that the base sticks out is the amount the bullet has to be in front of the case. I've now done 3 rifles that way and I've not had to reset my breech seater!

The other way is to install a bullet some distance, knock it out and measure how much was Engraved by the rifling, move the seater until you have more than half the base band engraved.

10 gr of Unique will get you started with a ccurate load at about 1000 fps with your 300 gr bullet.

Frank

The resistance that I speak of, is the resistance of the bullet in the rifling, when it's partially BSed.

The rod can be anything, including a piece of wooden dowel but, it has to be measured accurately.

I never used a wad with that load (10 gr Unique) and never spilled any powder. It was my 2nd most accurate load for 45/70 (430 gr bullet) and it takes up very little space in the case. You would actually have to tip the case down to spill any. If you wanted to use a wad, only put it in the case mouth.