This model should be able to fit you double stack magazine.
Yes, this should work on the BuckMark magazines.
It should, however it is not a guarantee.
That is dependent on whether it is a single or double stack magazine. The ST3 is meant to be used with most single stack magazines. This is the compatibility chart provided by ADCO:
"Most Single-Stack Magazines::
Colt - 1911 and copies
Kahr - 9mm & 40
Kimber - 45 single stack, Micro 9
Ruger - P345, LC9, P97
Remington - R51, RM380
Rock Island Gov’t
S&W - 39, 52 & Shields
Sig - 220, 225, 239, 938
Springfield - XDS single stack
Walther - CCP, P1, P5, P38, PPQ, PPS"
Yes, this should work. I had the same problem with my Millennium G2, it got slightly better when I stored the magazines fully loaded for a few months (but I still use a loader most times).