Horn and whistle signals, are rarely used except at crossings or in MW work areas. In fact use of the horn is prohibited in certain areas, per Railroad policy, or local government operation. These signals were meant to alert crew and other railroad workers of the impending movement of a train. However, since the inception of automatic block, radio communication and other modern conveniences, the use of horns is situational, and less frequent.
The front of the locomotive is designated by a small F. That applies whether or not the leading end of the locomotive is the F end. Push pull cab cars are considered locomotives, even though the are generally not powered. They come under the same general rules and practices as locomotives, vs. passenger train cars. In practical terms, the direction of the locomotive for hand signal purposes is determined by the direction of the train, and / or crew job briefing, where points of confusion are resolved.
Being that we are talking about the railroad industry, all of this is determined by a complex intermingling of laws, regulations and policies that vary from location to location, road to road, and country to country. That's why all operating employees are required to be qualified not only on the rules, but the special instructions, that modify the rules to conform to local practices. When I retired, nine years ago, most railroads in the Northeast, used NORAC Rules, while Railroads in other parts of the us used either GCOR or Proprietary Rules established by individual carriers.
From a model railroad standpoint, do what suits you...