Is it safe to use a 19V DC adapter for a 12V input if it can provide enough watts?
No, don't use it! The PSU will supply 19V ("voltage source") and will probably damage the load. There are PSUs that limit output current by dropping/regulating the output voltage (which then can be considered a "current source"), but as long as the required current isn't constant that wouldn't work either.
Most devices are designed for a specific operating voltage and sink current respective to their task. E.g. in standby mode your TV will sink a low current, but when the display is on, it will need much more.
The power spec "35W" just says, how much power it can provide, but at a higher voltage the load most probably will sink more current. More current at higher voltage means much higher power, which might destroy the load.