RG:
The Ari Zal states to have completely different clothing for Shabbos and weekday, including a tallis. Whether the appearance is the same is not mentioned. As for Shalosh Seudos with Divrei Torah, that is the Chabad line. It is unique, and is not accepted elsewhere, not among Poskim, not among Chassidim elsewhere. Being yotzeh with Divrei Torah is not a lechatchila according anyone besides Chabad. Your quip about Hayom Lo is also not accepted anywhere else. You can push the Chabad agenda, but the CR is not the place for it. It is only referenced in halacha with regards to Erev Pesach. I suppose one could argue that someone that cannot eat for other reasons might at least substitute with Divrei Torah, but I would look to a Posek to support that. The Rav Shulchan Aruch does not bring down about substituting Divrei Torah for food at Shalosh Seudos.
In terms of what is Shabbosdige food, it is challenging to make most across the board statements. Can one use a can of tuna for fish at the first 2 Shabbos meals? How about sardines? Perhaps suchi? Our answers to this will be tough to put into anything specific in halacha, and it more dependent on our personal judgment. That leaves it extremely variable to cultural differences. There are foods that were common in different communities, with Sefardim and Ashkenazim being only one such difference.
The fish and milk issue is actually interesting. It seems that the source for this is a typo, in which the posek was referring to the sakanah involved in eating fish together with meat, and either wrote or the typesetter wrote cholov instead of bassar. This custom is not widely accepted, and the majority of the frum community eats lox with cream cheese, for example. It is inaccurate and unfair to refer to this as being machmir or meikil. I have discussed this issue with rabbonim and poskim. Not a single one agreed that there is “chumrah” to not eat fish with dairy.
Lastly, it is noted that the Shulchan Aruch states that the minhag on Shavuos is to eat מאכלי חלב, and there are numerous reasons given. There is only one that can be interpreted to refer to cheese, the posuk in Tehillim that calls Har Sinai ההר גבנונים. Interesting that in Hilchos Chanukah, it states clearly that the custom is גבינה, and there is no direct reference in SA to milk. The MB states that Yehudis gave the general cheese. There is a nice pilpul for this.
There is a famous vort from the Chiddushei Harim where he states that every day of the year corresponds to one of the Mitzvos Lo Saaseh. He references that Tish’a B’Av corresponds to Gid Hanosheh. He states that Shavuos corresponds to the issue of bassar b’cholov. In order to demonstrate that we are following this mitzvah, we eat both dairy and meat, and wait the requisite time between them.