If /'/ is to be kept distinct from /x/, /'/ must be [T], giving [aTua]
for /a'ua/, which is unlikely to become [awa].
I'm not sure if this is meant to be an anglophone or a universal claim.
Anglophones tend to render [x] as [k], as in _loch, bach, Bach_, and
Germans have no problem distinguishing /h/ and /x/ systematically,
though it's arguable that there are no [h] : [x] minimal pairs, as [h]
is only in onsets whereas /x/ in onsets is realized (in the standard
accent, at least) as /C/.
It would be an assimilation rather than a fortition. As I've said
before, [h] is articulatorily impossible as a realization of /'/
in some environments, e.g. /i'i/, at ordinary speech rates,
I articulate /i'i/ as [iCi], /u'u/ as [uWu], /ii/ as /j\i/ (with a voiced
palatal fricative like Spanish-Spanish "y"), and /uu/ as [wu].
Obviously it was the glideless /ae, ea, aa/ type that led to Lojban's
"'". That in itself was not so bad a move, tho the choice of realization
was, but making it contrastive with zero between other vowels gives
greater headaches. I'd have just forbidden them altogether; going all
Livagian on their ass, I'd allow i to be followed by any vowel but i,
u to be followed by any vowel but u, e to be followed by no vowel but i,
o to be followed by no vowel but u, and a to be followed by no vowel
but i and u.
This would, of course, involve a complete discarding of the cmavo list and
starting over.