That breechplug is threaded, and that does have to be removed- the bolt slides straight out of the back.
Suggest you let it Kroil soak in for a day, try again. It should unscrew with finger pressure. Most of my Mossys are older, and use acone shaped external breech plug, but they changed to the one with the tapered plastic tail on yours for looks.
If KROIL does not do the job, I am going to suggest another forum- rimfirecentral.com There is a Mossberg section, and one of the regulars there may have a 352K. Tell 'em C3 sent ya.