How can I allow guests to manually control lamps under wifi control?
Google Home is nice, but you can't control primary room light exclusively with it. 

The building codes absolutely require that some lighting in each room be controlled by plain wall switches in standard locations. The locations of the mandatory switches are defined by Code.  The brightness level is defined by Code, though in some living spaces it is allowed to switch a receptacle and the citizen is on his own recognizance to plug in a workable light.

As long as there is a light switch in the proper location, and it operates some light in the normal way, the Code requirement is satisfied. You can have as many other lights as you please, control them any way you please, and you need never use the statutory light switch.

Lack of legal lighting could endanger your occupancy permit and thus, your mortgage.  What's more, if an accident is caused by someone being unable to turn on the light, the insurer will cite the defect as reason to refuse coverage. Your mortgage lender will have a problem with that too. 

On top of this, that Sonoff junk is a direct import from China and uses this loophole to bypass all our safety regulations, that's why it's cheaper.  As such, it is not approved by a Nationally Recognized Testing Laboratory (NEC 110.2 on the very first page) so there goes your fire insurance.  And again if the inspector catches this in your home, he'll make you tear it all out.  Rules are eased somewhat for smart devices which are not made part of the house wiring and are merely plugged in.