Time travel is only for DLC.
Also, if you want Chrom to marry someone, you need to do it before you defeat Gangrel. Otherwise he just gets married to some random woman.
There is a slight time jump to the next plot point after Gangrel, but it's nothing big.
Also, this isn't a spoiler, just something I noticed.
"Marth" is right, she did complete her mission to change the future, but she hasn't yet gotten the result she wanted.
Remember that assassin that she defeated? It was supposed to gravely wound Chrom, now that he's dead, "Marth" did save Chrom from being injured.