RE: Grid snap sizes: pixels per unreal unit setting affects how texture resolution maps to units. You can adjust value for specific sprites or all existing sprites using property matrix, and you can set project-wide default for new sprites in Paper2D project settings.
Some parts of how to place camera depend on your desired behavior (do you prefer to allow more/less of level to be viewed depending on aspect ratio or would you prefer black bars on other devices). general gist is to define a nominal aspect ratio and pixel resolution and work math out at other resolutions.
For example, you might define that your source screen is 256x192 pixels and your nominal aspect ratio is 4:3. Thus at 1024x768, you want to set it up so each game pixel shows up as a 4x4 display pixel. If screen is instead 1280x720 (16:9), then math isn’t as nice and you have to decide how to adapt.
Scenario 1: Use 3x3 pixels (nominal takes up 768x568), and now do you show more field of view (e.g., actually showing ~426x240 pixels), or do you add a black border
Scenario 2: Use 4x4 pixels (1024x768), and now do you show more in X and less in Y (e.g., actually showing 320x180), or do you show less in Y and a black border in X
etc…
Based on how you decide to proceed, you choose OrthoWidth on camera to match your actual X resolution in unreal units (not pixels, so in 1024x768 case above, with pixels per UU of 1.28, you’d want display to be 256/1.28 game units wide, setting ortho width to 200).
Sadly it gets really messy if you want to support a wide range of aspect ratios and still have nice crisp pixels, but that should get you started.
Cheers,