Please read my other posts.
1。国内的工作证明怎么出具,是要经过公证的还是单位盖章就可以?
If you can, 公证 will be great.
2。3年前出国前办的各种公证是否还可以使用?
Yes
3。一般被批准以后通常要在多久时间内landing? 去landing
的时候返回美国是否需要签证?
Within one year from your medical exam.
You don't need a new visa and I94 if you if you visit Canada less than 30 days from USA