Regarding question 2, if you connect a 50 ohm generator to a 50 ohm load, the junction node impedance will be 25 ohms. Nothing specific about attenuators.
Building attenuator networks for the VHF or even UHF range is no rocket technology, but you should be aware of parasitic component parameters and know how to estimate it. Preferably have a network analyzer to verify the results.
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The purpose of the -60 dB attenuator isn't quite clear. Most spectrum analyzers have 20 dBm (0.1 W) or even 30 dBm (1 W) input rating by internal attenuator networks. When working with 1 W level equipment, an external 20 dB attenuator may be useful as protection means, but 60 dB would be useless. It's a different matter, if you are measuring high power transmitters, but then you are looking for higher attenuator rating.