As others have said, allowing rebinding of the controls is always a good idea. But, ideally, your hotkey system should tie into your GUI. A good GUI/Hotkey pairing makes things intuitive, and thus, they should influence each-other. So without knowing how your UI is setup, It's hard to give you an answer on which would work best for your game. So I'll give some examples.
In most ARPGs (Torchlight, for example), the hotbar is a horizontal menu, which has a similar layout to the row of number keys on the keyboard. So, the number keys act as a one-to-one representation of your hotbar (the '1' key is the 1st option on your hotbar, and so on). The number keys are easy to reach, and it's intuitive to understand what you press correlates to what you'll get. If the hotbar wasn't displayed this way (say, as circular menu or a list), the number keys would be much less intuitive to use. Sure, for some, it can be annoying to click the higher numbers, but that's where rebinding comes in. Players can rebind to make the hotkeys more comfortable to them, but the default layout helps the player wrap their head around how the system works. In addition, usually the most used items are lower on the bar, since those keys are easier to reach. Plus, the hotbar layout works well with a mouse, which is good for newer players who aren't comfortable with using hotkeys.
The other thing is, it's nice to have context sensitive buttons. If your game has sub-menus or the menu has a bunch of "hotkeyable" options, it's nice to have a uniform system for selecting things. A great example is Age Of Empires 2's hotkey system. The menu options are setup on a grid that represents the player's keyboard. The first 4 options are selected with 'qwer', the middle four 'asdf,' and the last row with 'zxcv' The game's GUI makes it intuitive to understand and keep track of what keys to press by just looking at menu, just click the corresponding key for the option's position on the grid. So, if I want to grab the stone mining upgrade, I select a Mining Camp, and then press 'E', since it's the second option on the top row of the camp's menu. If I want a villager to build a farm, I select 'Q' to open the economy build list (first option, first row), and then 'A' (first option on second row) to select a farm. Plus, again, it's still easy to use the menus with a mouse (but much, much less optimal). The player can even hover over an option to see what it does if they're not sure. For additional clarity, you can even have a little icon displaying what key to press on the icon.
You couldn't really do a system like this as intuitively with the mnemonic button layout, since the keys would be all over the place, and there's a lot of different options for different units and buildings. But, it can be done. Most text editors use a mnemonic system for their tool bars. Alt to start selection of the tool bar, then the first letter of the drop down menu's name, then the first letter for the option. The menus usually even highlight the letter in the option's name that corresponds to the hotkey, to signify what to press. But even then, the most used shortcuts make don't make much "mnemonic sense."
Ctrl+c makes sense for copy, but ctrl+v doesn't make much sense for paste. Ctrl+Z for undo and shift+ctrl+Z for redo don't really correspond to anything either. Both cut/paste and undo/redo shortcuts are placed next to each-other because they're used together all the time, and they're placed in a position out of convenience and ease of use, since they're used extensively. And, because they're used so much, they're very easy to remember regardless of what they are. The tool bar selection makes it easier to dump a bunch of infrequently used options in a quick to find way, but the more useful options are placed out of ergonomics.
But, this leads to the problem of overlap. You could, for example, have 'U' as the universal button for "Upgrade unit," but what if there's multiple upgrades? Or what if multiple things make sense for the same letter? In your own example, 'P' for 'Party Members', and 'P' for 'Parliament' causes an issue. You could have 'M' for 'party Members,' but what about 'Map?' The more options, the less intuitive it becomes. The letter highlighting helps, but it defeats the purpose of mnemonic layout making it easy to remember, since you're scanning the vertical list for what to press. The toolbar could just as easily have random keys to represent the options, and it'd probably work just as well.
In addition, you could implement a system that allows key rebinding for each individual sub menu, but it can get unwieldy pretty quickly, but can be a good option to give the player if possible. Also, It's not as big of a deal, since it's turn based, but for mouse heavy games, it can get pretty tedious to have to constantly move your left hand around the keyboard for different options, since your right is preoccupied with using the mouse. Which brings up an important point on rebinding. Rebinding isn't important just for comfort, but accessibility as well. Just because the layout works well for you, doesn't mean it's a one size fits all option, especially for those with disabilities.
Anyway, I think I'll end it here, I wrote way more then I was planning to, but I hope it helps
In Summary:
- Make sure the default hotkeys and GUI complement each other.
- The more an option is used, the more comfort wins out.
- It's easy to remember a shortcut you use all the time anyway. Even if it doesn't "make sense."
- The more complex the system, the more the mnemonic layout breaks down
- And the less intuitive it becomes.
- Always allow for key rebinding.
- There's no excuse if you're using an engine that makes it easy to do.