constraints on wanna

<<I often pronounce "wanna" as ["wQ:nu] before vowels.>>

That's interesting. "wanna" is always [w@n@] for me, even before vowels. I would pronounce "I wanna eat now" as [aI w@n@ it naU]. I've never heard of the version with the unreduced vowel used in any context. By the way, do you also have version of "gonna" and "gotta" with the unreduced vowel, i.e. [g@nu] and [gQ4u]?

Yes, I have [g@nu] and [gQ:4u] before vowels..

I am different in that when I have an unreduced final vowel I usually use full "going to" ["go:1~Nt_j}_^u:] and "got to" ["ga?t_j:}_^u:], rather than shorter forms like Lazar's. Also, my use of reduced versus unreduced final vowels is not conditioned by phonetic environment but rather by stress/emphasis and register, with higher registers tending to favor unreduced final vowels.

See how useful such "sloppy" language can be:

Who do you want to visit? (AMBIGUOUS)

a. I want to visit Mr Pedantic.
b. I want Mr Pedantic to visit.

Who do you wanna visit? (UNAMBIGUOUS)

I want to/wanna visit Mr Pedantic.
*I want Mr Pedantic to visit.

"Wanna" can help us avoid ambiguity in some areas of usage.

Hello old chap,

<"Wanna" can help us avoid ambiguity in some areas of usage.>

I doubt very much whether any real ambiguity would exist, in context.

Do you have an example where there would be real ambiguity?

MrP

<Do you have an example where there would be real ambiguity? >

In the one I've just given, even though you, personally, do not think it suitable.

''
"He doesn't want to"

"wanna" cannot be used at the end of a sentence. Hence, you can't say *"He doesn't wanna" and likewise can't say *"He's not gonna".''

GONNA can be used at the end of a sentence:

Did you tell Al?
-I was gonna...

<<"wanna" cannot be used at the end of a sentence.>>

And 'wanna' can be used as well:

Dad--"Son, you have to clean your room."
Son--"I don't wanna!"