1) Why are you using ParameterChar? It has been obsolete for at least 12 years; it was added many years ago to ease transition between 4DOS and 4NT batch files, and changing it now only makes your batch files incompatible with everybody else (and vice versa).
2) Why did you choose that particular character? It's one of a handful in that character set that don't translate between code pages. (Each code page and font combination have between 0 and 10 characters that are mangled by Windows during translation to & from Unicode; your 0xF8 is one of them.)
The questions wasn't to me, but here goes.
1. ParameterChar value seems to be controlled by Advanced/Parameter in the option dialog, so maybe you should ask "why does that dialog use that value".
2. This one is probably quite easy to explain. On the nordic keyboards (at least on mine) you'll have to hold shift and press the key having ^, then press space, before it actually shows (the space char isn't typed/printed). ø is a single keypress without modifiers, and it is one of those characters you (almost) never encounter in executable filenames, commands etc.
In other words, it won't clash with anything.
The lower keyboard in the linked post
seems to be the pan-nordic layout.