I think when you take them out, you have to wash it under tap (don't turn tap too strong) and clean out the gypsum and dolomite. After the roots have been clean, repair the seed tray with cononut coir mix (as you've got) but I used sphagnum when I grew them.... Just leave them stay nicely on the top of the mix, try to hang them stand balancely (but do not need anything hold/tidy them)
The hardest thing that I fail to grow them that I can't not create the appropriate humidity for them. Also should not water too much as it will destroy their roots.
I can not explain for you everything but normally growing seedling is not very easy, it requires some skills and you have to observe the weather around you. Other environment factors should be concerns. Different grower got different way to apply on their growing, also it vary from country to country depend on geographic, weather and environment.
Good luck my friends