Is a Mikavah process required per Halahah for the conversion process? With no Mikvah process, can the conversion certificate be issued? Is that certificate valid? Can that person observe Shabbat as a Jew?
Yevamos 46a - b אמר ר' חייא בר אבא אמר ר' יוחנן לעולם אין גר עד שימול ויטבול וכיון דלא טביל נכרי הוא - a person isn't a convert until he is circumcised and ritually immerse
This is how we pasken in shulchan aruch (yore dea 268,1) like the tanna Rabbi Yossi Although the premise to the question is justified as the tanna Rabbi Eliezer says mila (circumcision) on its own is enough without mikva for a man (See Yevamos 46a-b).
one needs 3 chachomim to oversee the immersion and until then the pending convert should do 1 discreet melacha every shabbos E.g carrying his wallet in his pocket in a public domain inorder not to transgress גוי ששבת – חייב מיתה, שנאמר "ויום ולילה לא ישבותו"- A gentile who abstains from melacha is liable to death as it says "day and night they shall not stop" (sanhedrin 58b)