MacOS – extend the press and hold app to other keys

I don't know a direct solution to your question: Reduced delay. If your iPhone is jailbroken you might be able to find the preference file that sets the delay and reduce it.

One work around could be to not deal with the accents at all and have the iPhone insert them for you, as autocorrected words. You can now type full speed.

To add custom words to the dictionary:

• Add the Japanese Ten Key keyboard: Settings > General > Keyboards > International Keyboards > Add New Keyboard > Japanese Ten Key.
• Add words: Settings > General > Keyboards > Edit User Dictionary.
• Have the Word be what you want to type w/o accennts, and the Yomi be the accented work.

For example, I added Word=fooxx and Yomi=föoxx, I then went to the Notepad and using the English keyboard I typed "fooxx", and the iPhone suggested föoxx. When I did this example without the "xx" it did not work, perhaps because foo is already in my dictionary.

Alternatively, if you iPhone is jailbroken you could try using Xpandr

The easiest way might be to disable the Fn key. Since you say that using Fn+F10 works, you could go into System Preferences > Keyboard > Keyboard, and enable Use all F1, F2, etc. keys as standard function keys. This way you no longer need to press Fn to send F10.

The second solution is on the PC side. You could install a utility such as AutoHotkey on your PC and remap the keys there. That way the keybindings will only work on the PC. Lifehacker has nice looking tutorial about this. In AutoHotkey, you can decide which applications the shortcut applies to, but in your case it seems like a global hotkey would work since it is running on the PC side.

Another solution might be to try one of the ways to create keyboard shortcuts menitoned in this Lifehacker article. This article's goal is to help you do similar things that AutoHotkey let's you do on the Mac.