I think the suckers are just typical for crabapples, doing the same with mine and it isn't hollow. I have a regular apple tree that's very hollow, and leaning almost on a 45, looks like it grew that way. I'm at work so I can't take a pic, it's knarly. It has had to been hollow for many years, we've lived here five and it produces tons of apples. Apple trees are tough. I pick apples off hollow wild ones that have been that way for over 20yrs.