I would go with option 1 (embedding lambda within alpha) as it is the easiest to maintain / most performant option, however I would use golang plugin infrastructure, so the lambda logic would be loaded only if/when a schema with @lambda is loaded in (see Writing Modular Go Programs with Plugins | by Vladimir Vivien | Learning the Go Programming Language | Medium) , so when you do not need lambda functionality, you avoid loading (potentially big) unnecessary code.
Then you can have different implementation within the plugin (pool of goroutines, launching external process, using remote services etc).
If I was to write an embedded lambda server for alpha (and I might one day), I would like to use a plugin with goper-lua engine (GitHub - yuin/gopher-lua: GopherLua: VM and compiler for Lua in Go) and lua scripts. Then I could use the same “entry points” (golang plugin with defined interface).