The actual tense part can't be a debate, indeed. In Greek -mai is simply only the ending in present tense singular first person passive voice.
I have to suppose that the translations where they used past did so because they didn't feel like including any notes about either historic present tense or (as appears equally or more likely) subjunctive present tense.
As
@Moff Jerjerrod posts, the meaning of the verse would be that the (co) crucifixion is continuous, not something which happened at some point in the past and then changed. If subjunctive it also means that it is optional (ie if you have faith, you too can do this, etc).
As a side-note, the original has συνεσταύρωμαι as one word (with syn as a prefix), which of course doesn't allow any meaning of a phrase such as "
through Christ I am crucified", since that would use "syn" as its own term in the sentence: 'Syn Christo stauromai' etc.
I also briefly wondered if there is even a (granted, incredibly minute and virtually impossible) chance that the "e" in synestauromai was not there due to rules about well-known progressions of consonants in complex -prefix+core- terms that spawn a vowel (e in this case), but because it would be past
continuous (Greek past continuous often has the verb begin with an "e"), thus "I was being crucified with Christ".
But again, there is no past continuous in Greek (ancient or modern) that can end in -mai.
But by now we are moving in theoretical ground for little reason; I have to imagine that
@Plotinus (where is he, anyway, we are discussing this on account of him
) will have easy access to some scholar of Greek (even of that time period) to provide a definitive answer.
Anyway, found another site with endings of verbs (ancient Greek) :
https://www.greek-language.gr/digitalResources/ancient_greek/tools/verb_forms/annex.html
Relevant here is the -ωμαι ending (synestauromai, it is with an omega in Greek in that To Galateans verse), of which you (again) can read that it is in subjunctive present tense passive voice:
(Υπ stands for "Υποτακτική" = Subjunctive)