This will be purely an intuitive response and not a scholarly one ... but I'd say first off that if you really wanted answers to the proposed scenario you'd need to be more specific as to what tribe and what mask. Probably any and all possibilities has happened
somewhere in Africa at some point in time. If it were a funerary mask or a secret society mask it would probably not be up to the dancer as to the form and the making of the mask at all. Even the mask maker in some tribes is not the mask maker because he is the most artistic or because he chose the position but because the diviner says he is the mask maker...and the diviner says he is the mask maker because the 'spirits' say he is - and so it would be with the mask ... form following tradition. The mask most likely wouldn't even be a mask for the dancer -- but a mask for the people -- therefore personal preferences that have an 'I' in them (like 'hey I think it would be cool if I had a spiked crest on my kifwebe!') would not be considered or even thought. Perhaps if a dancer had a vision or a dream and discussed it with the 'diviner' and the diviner spent weeks in the deep jungle
living on nothing but grubs and wild honey talking to the 'spirits' then maybe a change could be incorporated if it was deemed good for the entire village. This could be dangerous for the diviner however because if the crops failed or misfortune fell on the village that year -- he could be blamed.
On the other hand -- perhaps some masquerade masks or celebratory masks of certain ceremonies or peoples have other requirements and allow for less restricted possibilities. In the purest tribal tradition the focus is on the people -- not the personal -- in some tribal cultures there is not even a word for 'I' just as there is no word for 'art' in some. Imagine if every time you said 'I' that you used the word 'we' instead -- and not only in your speech but in your thoughts until it was so deep in your psyche that there would be a complete paradigm shift in how you viewed the world. It would be through these eyes that
glimpses can be imagined in answering your scenario's questions.
Some masks may require going to a carver 50 miles away ... maybe not because he is a better carver than the one in the village but because he survived a viper bite last hunt or because a meteorite streaked across the sky to the south where that carver lived at precisely the same time the elders spoke around a fire about having this mask made. Or maybe they farm it out because the carver has been embroiled in a scandalous tryst involving the King's 1st wife's daughter and his 2nd wife's cousin. Even tradition is altered at times out of necessity and/or circumstance. Life is not static ... and neither are a people that live close to it's pulses in my opinion.
As far as using a model ... by the time a carver is making masks for dances it is so ingrained in his entire being that there is no need
for a model. I recently bought a Hopi pot from a great granddaughter of Nampeyo -- one of the most renowned of Hopi potters -- who, by the way, kept making pots years after she was blind. letting her daughters do the painting on the pots. The woman I bought the pot from doesn't use a pencil to lay out a design -- she does it all by heart because the design and the meaning and the love of her ancestors all live within her and guide her hand.
What if a Christian wanted to have a cross made? A cross is what it is -- an accepted symbol ... you can't really go to the jeweler and say ... 'ya know ... I'd really like a cross with the short section on the bottom instead of the top'... It would be like hanging Jesus from his feet ... all the symbolism lost ... and not a cross at all. Not to say that every cross looks alike ... within certain parameters there can be many variations. And would you go buy
your cross at Walmart? Or a fine jeweler? Or perhaps have it custom made? All depends. What if Christ had been crucified in the 20th century -- would people be wearing electric chairs around their necks? A mask is not just a mask ... it holds meaning and is tied to a collective consciousness along with stories and songs reaching down into the unconscious. Once a symbol -- or a mask has reached such a status it has a life of it's own... a 'power' given to it through the symbolism of invested beliefs.
Symbols, secret societies and cults do die off ... and new ones appear as changes occur and are incorporated into a culture's identity. Death and birth are inherent in all things and it is left to those that survive to hold or discard what may or may not be deemed as valuable to the continuity of a culture. It is a dance in itself to find the balance