Indeed, the symbol Script A (or Latin Alpha or whatever else you may call it) <ɑ> and the Length Mark <ː> are not displayed properly with MS IE 5 (though it's OK with version 4 and Netscape 4 or later). I don't think there's a cure for this (Script A would only appear in certain combinations, e.g. with an Ash preceding it <æɑ>) and substitutes ARE needed.
The colon <:>
is quite a customary typographical substitute for the length mark so no
one could possibly blame us for using it.
As to the other gremlin, too, employing <a> for a back(-to-central) vowel won't be a precedent either, though it might bring about some confusion (After all, the non-EFL division of OUP has adopted <a> for the vowel of cat).
What I suggest is that we substitute normal Greek Alpha <α> (decimal numeric entity α) for the treacherous one.
Re: The red G-like character.
I can't see it with OutlookExpress and IE 5 but there's no problem with
It is, however, the same as the normal Lower-case G in Lucida SU; it is included there just in order to avoid replacing it with Looptail G (which is not recommended for "voiced velar plosive" by the IPA) when you switch to a serifed font (say, Times New Roman), so there's no use of it as long as we stick to Lucida SU.