Some of the vias come too close to SMD solder pads and are too large. Make them smaller and place two or three in parallel. This will reduce inductance a lot.

Concerning the LM25574, take the layout the manufacturer recommends in datasheet. If he tells that a solid ground plane is "recommended", then you need one!

If the switcher is farer away from sensitive parts and edges of board, the ground plane of switcher can share the common ground plane. But in any case you have to expect the introduce of common mode noise.

You can provide a local ground plane for the switcher. But then this must involve all layers and you need a quiet ground point you can connect to the common ground plane. This could be the ground pin of output cap. But this would mean that you put the input cap near the output cap and/or that you use some sort of input filtering by the help of input choke or soft ferrite (depends on switching frequency) to reduce the HF input currents flowing across the "quiet" ground connection.

Usually you need an additional ripple filter at output.

Kai