al tifrosh min ha'tzibur

The Chayei Adam points out what seems to be a stirah in MG"A.  In hil Tisha b'Av the MG"A writes that if you are in the middle of eating a fleishig shalosh seudos and shavua she'chal bo starts that night (assuming you only stop eating fleishig during shavua she'chal bo or rosh chodesh falls on Sunday), you have to stop eating once the tzibur starts maariv.  You can't march to your own drum and extend your meal and your shabbos until whenever you want.  However, in hil tefillin (siman 30) the MG"A writes that if you have not yet put on tefillin that day, even if the tzibur davens maariv, you can still do your own thing -- so long as it is still not really night, you can put on tefillin.  Why, asks Chayei Adam, is night defined based on when the tzibur davens maariv with respect to the start of the 9 days, but not with respect to the last zman to put on tefillin?  

(I keep feeling like I am missing something in the question.  I would say that when you stand to lose a mitzvah d'oraysa like tefilin, then we are lenient and the individual is not schlepped along with the tzibur.  However, if all you stand to lose is an extra bite of fleishig, then we say stop eating.  On the other hand, I guess one could also argue that if one is not forced to be part of the tzibur when it comes to the issur derabbanan of putting tefillin on at night, then kal v'chomer one should be able to do what one wants when dealing with only a minhag like not eating meat during the 9 days.  The Chayei Adam offers neither sevara, just the question.)

In the Sha'ar haTziyun the M"B buries a teirutz to this question that is probably more important as a matter of hashkafa than a matter of halacha (the view of MG"A is not accepted).  He answers that ain hachi nami, just like with respect to tefillin, if you don't want to daven maariv yet then it's not night, so too with respect to fleishig, for you it's not night yet.  However, even if it's not night -- even if shavua she'chal bo has not officially started for you -- since for the tzibur it has started, since they are in more intense aveilus, you have a chiyuv to be mishtatef in their pain.  A person cannot be poreish from the tzibur when they are suffering!

Maybe this why the Rama paskens that the minhag is for pregnant or nursing mothers to fast on a regular fast day unless or until they feel unwell or incapable of doing so.  My brother asked me the following question: if Chazal never included pregnant women or nusing mothers in the takanah to fast, then what do they gain by doing so?  I would say that that argument holds water viz a viz the chiyuv ta'anis, but there is an additional chiyuv here of not being poresh min ha'tzibur.  Even if you are exempt -- for you it's not the 9 days yet, it's not shavua she'chal bo, you are not included in the chiyuv ta'anis -- since the tzibur has a chiyuv, you have to do something to show that you empathize with their pain and share in their suffering.