Match impedance and net dimensions of RF power amplifier
How can I extrapolate the allowed net characteristics (Width), while the only thing known is the maximum VSWR?


You could simulate your layout to estimate the VSWR the chip will be seeing.

You'll need to consider the reflections from the layout, layout variability, and the antenna and its connector to determine the worst case VSWR your design will produce.

Note that the 5:1 maximum VSWR specified in the datasheet represents a 67% reflection. It should be fairly straightforward to achieve better than this spec at 450 MHz. 


  Not to mention the input: it is a sort of CLC T-network, but I'm totally unaware of the input impedance of the chip,


This one I'm stumped on. The datasheet doesn't specify any input impedance or VSWR that I can see. The equivalent circuit they show implies a low-impedance input. You may want to call your sales rep to solve this one.