Zelenskyy: collaborators to be convicted according to the law of Ukraine
Zenenskyy's speech on 19/Sep (video/UA, transcript/UA) seems to answer your question.
Translation and highlight mine:
[…] The collaborators, similarly, have {only two} options: either they try to flee (and it's an open question whether the russia lets them in, even for it gave them its passports), or they will be convicted according to the law of Ukraine.
By the way, I want to thank the National Police for quick restoring law and order in the newly liberated territory.
So yes, there are only two options, both assume that the prosecution should be lawful.
Note that establishing law and order after the liberation is a crucial precondition for the legitimate government were able to prevent crimes against the collaborators, and this is why Zelenskyy speaks about police restoring law and order.
Ukraine is fighting for the rule of law
For example, 2-3 months back the BBC publicized a few instances of alleged abuses of Russian POWs. Zelenskky's response was that such cases were against policy, would be looked into, and would be punished if confirmed.
Yes. The whole idea of Revolution of Dignity was about Ukraine's attempt to become closer to the civilized world. This was exactly what russia has attempted to prevent by its armed invasion, and this is what Ukraine is fighting for today, the European values, the rule of law. Any transgression, real or suspected, against the POW would be against the international law and so it would undermine the whole purpose of Ukraine's liberation movement. In addition, rest assured, such cases would be quickly inflated to jeopardize the international support to Ukraine.
When Ukraine can take the responsibility for legal prosecution of collaborators
Note, however, that the international law makes clear difference between POW (and crimes against those, by a government including its armed forces), like in case you have mentioned, and guerilla war (in which civilians fight against the enemy).
- during the period of occupation, any civil resistance, including the execution of russians and collaborators, is considered guerrilla war, hence not prosecuted by Ukrainian law.
- after the liberation, the defending state is only responsible after its legitimate government begins exercising full power. The "grey zone" could extend up to one year, see below.
I would recommend the interview of Mykola Holomsha, Ukraine's ex-Deputy Prosecutor General (video/UA, transcript/UA) who explains it in simple words.
Fifty shades of collaboration
[…] some Ukrainians collaborated with the Russians, or at least accepted jobs to feed themselves and their families.
Accepting a job to feed one's family is neither equivalent to collaboration nor to high treason.
Article 111-1 of Criminal Code of Ukraine (hereinafter referred to as CCU) includes in the definition of collaboration such activities performed by a Ukrainian citizen as (in terms of jobs) cooperation with the aggressor state, its armed formations, or its occupation administration. (Review in English; Full text in Ukrainian)
Simply speaking, a person who's baking bread or selling goods in a store is likely not committing a crime.
However, someone who "accepts a job" of a russian polizei clearly aids the aggressor's armed formations. Someone who agrees to teach Ukrainian children that there is no Ukraine, and the Ukrainians are just spoiled russians who should be "healed" (denationalized), promotes the enemy's agenda. Such people are, indeed, in trouble.
In addition, the CCU’s Article 40 details exceptions to a criminal action because of physical violence or mental coercion, like threats or physical violence, including directed to relatives.
Time considerations
From past experiences, such as post-liberation France in 1944, there is a risk of summary justice being handed out […]
The source you linked, along with others I've read, mention that lynching, voluntary expulsions, and even executions of collaborators has occurred within a short period, primarily before the legitimate government institutions began exercising the functions of government (read: police, courts, prisons, etc.):
the historians have restricted it to a conceptual cage of a few months of high emotional drama during the liberation — Megan Koreman: The Collaborator's Penance: The Local Purge, 1944-5
The first phase (épuration sauvage) consisted of popular convictions, summary executions, and the shaving of women's heads. Estimates by police prefects made in 1948 and 1952 were that as many as six thousand executions occurred before the liberation of France, and four thousand thereafter. — Wikipedia
I believe, this influenced the future laws made included a clear definition of period of occupation, during which the occupant is responsible for crimes. Article 6 of the Geneva Convention defines End Of Occupation as follows:
In the case of occupied territory, the application of the present Convention shall cease one year after the general close of military operations; however, the Occupying Power shall be bound, during the duration of the occupation, to the extent that such Power exercises the functions of government in such territory, […].”
