While I see your point, there are already a multitude of rules that weaken other roles in the pursuit of more balanced and immersive gameplay.
Blocking off extra entrances works against raiding classes, PD, and Hitmen, but is accepted because it balances gameplay for mayors and crack manufacturers alike.
"Do not set up an unfair advantage in a firefight within your base" - Once again, weakens any class that can base but is made in the pursuit of fairness.
This suggestion is intended to balance Hitmen against other Hitmen. Requiring players to be damaged first when one-shot weapons exists, inadvertently encourages the same behaviors that other roles use to escape hits. I believe there is already a sense that when a hit is placed, that the hit may not be carried out. Hits against the mayor, police warden, AFK players, Admins in Sits, and basing classes are often difficult/ impossible to carry out, especially since the hitman class doesn't come equipped with raiding tools. These hits are frequently left to just expire.
To address your first point, there are benefits that each and every roll has that are inaccessible to other roles. This is just the nature of role based gameplay. As such, when roleplaying, it doesn't make sense to ask hitmen to ignore hits placed on them when they are clearly visible, just as you wouldn't ask mayor to ignore PD not in the PD party, or ask PD to ignore PD raids because of NLR.