I wouldn’t worry about snapping a bus to a grid.
Vehicles can be stuck wherever you want to put them.
Grid snapping is for modular pieces that need to interlock to create a bigger asset.
If you made the bus in interchangeable pieces with different grills, doors, windows etc. you would want it on the grid
For the light map you definitely had way too many islands in that example.
Like Darthviper said you want as few seams and islands as possible.
I like unwrapping light maps with a pelting tool. I use UVLayoutPro by Headus.
But I saw there is something similar now in Max 2015.
There is a bunch of different tools that do Pelting I am not recommending one or the other.
Pelting is like stretching the pelt of an animals skin to make it flat
but on your UV’s. It warps the UV’s to keep in in one island.
I would not use pelting for the main texture map of a hard surface object
like that bus. But for the Lightmap channel pelting is the approach I would use.
You can make the main body of that bus into 3 islands or less.
then do separate islands for the wheels.
You can scale the unseen faces on the back of the wheels way down in size.
21k triangles is an OK resolution for desktop or console.
These days I only like to develop for Mobile.
I try to keep my entire scene under 200,000 tris so
for mobile i would look at vehicles in the 2000 to 4000 tris range.
If you are planning on selling on the asset store, why not
make 2 versions.