I would have a go at doing Timber mate filling the crack, leave for at least 24 hours to fully cure & dry out before sanding back, then leave for another few days.
If the moisture/water content is too high when applying stain over timbermate it will rehydrate and turn to crap real fast.
There are a couple of ideas to contemplate:
1. very light, almost dry dabbing stain on with a small square of cotton - this may require a few attempts to allow each application to fully dry out.
2. use stain as the liquid required to turn firm timbermate into a filler slurry of the desired colour - you may need to be a couple of shades darker that what you think is necessary. If it turns out a fraction darker that stlll does the job of hiding light coloured veneer joins.
Cheers, Waz