I guess your problem is mostly coming from your understanding of the inputs of the epoching options (menu Import MEG/EEG). If your marker you use for epoching is the beginning of the epoch and not the stim (i.e. 100ms before the stim), just changing the "epoch time" from [-100ms,600ms] to [0ms,700ms] would fix your timing issue.
https://neuroimage.usc.edu/brainstorm/Tutorials/Epoching
If you want to fix the timing for existing files, use process "Pre-process > Add time offset".