Recommend you let a gunsmith do it.
The trigger is held by 2 screws and one pin that needs to be pressed out. See the link at the bottom of this page.
No. Trigger guards for these were made of plastic.
There is a website www.rediscovered-shooting-treasures.com/mossberg that offers copies of the original factory instruction sheet for the 810 models. This is the "manual" that came with your rifle and covers a lot of information including trigger adjustment.
I believe this rifle was made by Mossberg and is the Mossberg 46B. Do a search for photos of the 46B Mossberg to see if it is a match. If so, try Havlin Sales & Service as they may have a replacement trigger guard.
Do not attempt to use other than a Mossberg bolt made for that rifle.
Marlin made the 336 for Western Auto Revelation 103.450 & .451. mossberg also made their model 472 lever gun for Western Auto Revelation 205.
A Mossberg 22 rifle such as the Plinkster depending on the condition is worth around $150-200.
Cannot find an exact match- but if your trigger guard is black plastic- and especially if it has finger grooves in the tail of the guard- it was probably made by Mossberg. You can order replacement trigger guards from havlinsales.com There are 3 different trigger guards- check the photos to see if they match your rifle.
It is the rifling in a mossberg rifle barrel.
Your rifle was made for Montgomery Ward stores by Mossberg, and is a Mossberg 346K. You can check with a gunsmith, or check the website for gunpartscorp (look under Mossberg). Havlinsales also stocks many Mossberg parts.
There are dozens of different models of the Mossberg 22 rifle. You did not tell us which model, and we need that to give you an answer. Sorry.
With the exception of taking the action out of the stock, and the bolt out of the action- it is not necessary to disassemble the 146B. If you have taken more things apart, would suggest you take it to a gunsmith. There are no manuals for the 146B due to age.