Can you have a joint purim seuda and friday night meal if you started before skiah on the Friday. What are the implications - re continuing a meal into shabbos re benching, lighting candles, davening, not eating once you light candles until kiddush etc etc.
Rav Eliezer Melamed writes here:
[T]here is a custom to combine the Purim meal with the first Shabbat meal on Friday night. Some great Torah authorities follow this custom, while others recommend doing so only be-di’avad. In order to follow this custom, one must pray Minĥa and then begin the meal while it is still Purim. Then, around a half-hour before shki’a, one should accept Shabbat by lighting the Shabbat candles, place a covering over the bread, and recite kiddush over wine. Since one has already recited the berakha over wine (Ha-gafen) during the Purim meal, one should omit that berakha in kiddush. After kiddush, one continues the meal, making sure to eat a keveitza of bread, or at least a kezayit, for the Shabbat meal. At the end of the meal, one recites Retzei Ve-haĥalitzenu in Birkat Ha-mazon, adding Al Ha-nisim in the Ha-Raĥaman (“May the Merciful One”) section at the end of the prayer. After the meal, one prays Ma’ariv.
As couple of points are addressed by HaRav Chaim Bleier in his Chukei Chaim series on Parshas Tzav 5779 here
He writes there:
If one began his Purim seudoh during the day and wants to continue it into the night as his Shabbos seudoh, he must stop eating and make Kiddush close to bein hashmoshos before continuing since the chiyuv of Kiddush then takes effect (שו"ע סי' רע"א ס"ד)
As far as bensching he mentions:
Bentching - Retzei and Al HaNissim
- Did not eat bread at night. If one made a ‘poreis mapoh’ seudoh but did not eat after tzeis, some say he should say Retzei in bentching, but he should not repeat it if he forgets (שו"ע סי' רע"א ס"ו ומ"ב סקכ"ט ול); Others say he should not say Retzei (רמ"א שם).
- Ate at night. If one began his seudoh by day, continued into Shabbos night, and ate at night, all poskim agree he should say Retzei (מ"ב שם).
- Al Hanisim. If one says Retzei in bentching, he should not say Al Hanisim since that would be contradictory. Since Al Hanisim is not as crucial, he should only say Retzei (מ"ב סי' תרצ"ה סקט"ו).