breaking fast on coffee and cake on shabbos; chatzi shiur by peru u'revu, learning without rischa d'oraysa

1) Many people prefer to break their fast on coffee and cake rather than sit down to a heavy meal.  Is that an option when 10 Teves falls on Friday as it does this year?  Can you say kiddush and then have your coffee and cake and then eat seudas shabbos a bit later?  There are a few hurdles to overcome: 1) Those who do not drink a revi'is of wine from kiddush have a potential safeik bracha rishona on the coffee (Biur Halacha 174:2); 2) You have a potential safeik whether to say bracha achrona on the coffee or whether it would be exempted by birchas ha'mazon of the meal (MB 174:37); 3) Rama writes that seudas shabbos should be eaten immediately after kiddush, and ideally cake does not suffice for kiddush b'makom seudah at night (Shu"t Ein Yitzchak #12).  4) Al pi din one could wash, drink coffee, and then continue with the main fleishig meal (as MB says to do by Shavuos), but there is a Zohar that says not to have milchigs and fleishigs in the same meal.  Given all of the above probably best to ask someone in advance what to do or work out the issues here instead of waiting until you are about to sit down and eat.

2) The sefer Tzlach Rchav quotes a machlokes Chida and Chikrei Lev whether there is any value to a chatzi shiur of a mitzvas aseh, a topic we have discussed before (here, and earlier this year).  Perhaps one could argue that the safeik depends on the source for the issur of chatzi shiur (see Yoma 74a).  If it is learned out from a derasha on the words כל חלב, then the gezeiras ha'kasuv is perhaps limited to issurim like cheilev (there are Achronim who limit chatzi shiur even by issurim only to issurei achila).  If, however, the source is the sevara of כיון דחזי לאיצטרופי, the same sevara can be applied to mitzvos.  

Tzlach Rchav brings this up on the parsha because of a cute remez.  Among those who went down to Mitzrayim is the child of Dan,  וּבְנֵי דָן חֻשִׁים.  This is the only child of Dan.  What about the mitzvah of peru u'revu? The answer is Dan = דכר ונוקבא, and Chushim = חצי שׁיעור יחשׁב  מצוה.  The one child is a chatzi shiur of the mitzvah of peru urevu, but a chatzi shiur also counts as a mitzvah.

Regardless of how convinced you are by this remez, I think it's fair game to ask whether having one child can even be called a chatzi shiur.  The din of chatzi shiur applies to quantities, e.g. eating less than the required k'zayis of matzah.  You still ate matzah, just not enough. If, for example, you put tzitzis on three of the four corners of your garment, that's not a chatzi shiur.  You haven't done the mitzvah of tzitzis, just not enough of it -- you haven't accomplished the miztvah at all.  There is something qualitatively lacking in this case.  Having one child instead of two seems to me to be more akin to the tzitzis case than the matzah case.

3) When Yosef has a send off for his brothers to return home, he tells them  וַיְשַׁלַּח אֶת אֶחָיו וַיֵּלֵכוּ וַיֹּאמֶר אֲלֵהֶם אַל תִּרְגְּזוּ בַּדָּרֶךְ (45:24).  Rashi already alludes to a Midrashic interpretation that the Yalkut Shimoni fleshes out:

 ויאמר אלהם אל תרגזו בדרך א"ל יוסף לאחיו אל תתעסקו בדבר הלכה שמא תרגז עליכם הדרך. איני והאמר ר' אילעאי שני תלמידי חכמים המהלכים בדרך ואין ביניהם דברי תורה ראויין לישרף שנאמר ויהי המה הולכים הלוך ודבר טעמא דאיכא דבור הא ליכא דבור ראויין לישרף לא קשיא הא למיגרס הא לעיוני.

Yosef knew that if his brothers started learning in depth on the road home they would get carried away in the "rishca d'oraysa" of arguing back and forth with each other, so he told them to keep their learning light.

R' Baruch Mordechai Ezrachi asks: Why didn't Yosef just tell them that when they learn b'iyun, not to get carried away and to avoid battling back and forth over each point?  Why change the subject matter?  

The answer is that learning without "rischa d'oraysa," without battling back and forth, is not called עסק, is not called learning with depth and intensity.  The two by definition go hand in hand.