I’m not an expert either but my first thought was, that bright, opaque and pastel colors are the most difficult ones to start with because one needs a lot of layers to make it look even and this automatically results in rather thick layers that are brittle and chip easyly. In my experience shear shimmery polishes are easier to use because the shimmery pigments are more forgiving when it comes to inhomogeneities in application. Regarding the top coat you used it looks like it was just an unlucky coice and it seems to have absorbed something you where in contact with. Like dirt or strongly tinted food. You could try the scientific approach and use your topcoat on a blank nail and subject it to the same type of substance and see if it also gets this yellowish stain. If yes, the top coat is trash, if not it reacted with the polish underneath so you still have to find a different combo. It is also possible, that the top coat is overly sensitive to UV-light, is old or has this as a side effect of it’s polymerisation reaction. Also possible is, that your second layer was not entirely dry of did dissolve the layer underneath which may have caused the diffusion of components that where not supposed to hang out together. (But that is a bold theory).

In my experience several very thin layers are better than to few very thick layers and as already said, shear shimmery colors are easier to start with and the layers can be thinner

I would try a different top coat either way (possibly from the same brand, as this increases the likelyhood of compatibility)

Considering that white is among the most difficult colors (imo) you did an awesome job, though

Edit: now that i thought about it, it is possible that white and pastel colors are probably higly filled with mineral powder, making them very prone to chipping/flaking/breakinganfd that probably increased your desire to protect it with layers that were simply too thick