If you have any 110/115/120V appliances, they can't work on unsplit 220/230/240V, period. In theory they could be ran in series, but the reality is that series work only as long as both loads are perfectly same. If the load varies (as it always do) the overloaded leg will get less than nominal and the underloaded will get more - and those appliances are at risk of blowing. The proper way of running those is to use a transformer to convert 220 into 2x110. Depending on how many 110V appliances you have, it could be cheaper to simply replace them with 220V ones and remove the last remains of 110V wiring.
For connecting 2X0V mains, generator and solar together, it's impossible to tell not knowing your exact wiring. You should consult a local electrician, because 1-pole transfer switch doesn't sound right without clear grounding. Understanding your grounding is the key here and as @Harper says, Philippines grounding is a non-trivial one.
As you're currently describing it, I'd say that solar should use 2x110V inverters with 2-pole switch. But, it's also possible that you already have the 220V/2x110V transformer on the mains line and your contractor simply plans to hook up there.
The situation could be far more dangerous than you're describing. If 220V with 1 leg grounded gets connected to 2x110V center ground, at first everything may seem like it's working, but the moment the mains or generator goes live too, 110V will appear between "solar ground" and "generator ground". Again, it could still appear superficially working, but a person touching the generator or a solar panel casing will get electrocuted with 110V. That's why I can only advise consulting an electrician that fully understands both the Philippines 110 to 220V conversion issues in general and the wiring of your building in particular.