Is it permissible for the ordinary Muslim to follow a specific madhhab so that he can find a concession?

Firstly:

 We have previously stated on our website that the
breast-feeding which makes a person a mahram is five breast-feedings or
more, because of the hadeeth of the Mother of the Believers ‘Aa’ishah (may
Allah be pleased with her): Among the things that were revealed of the
Qur’an was that ten definite breast-feedings make a person a mahram, then
that was abrogated and replaced with five definite breast-feedings. Narrated
by Muslim (1452). 

We have also mentioned previously on our website that the
view that the testimony of a single woman may be accepted as confirmation of
breast-feeding is more likely to be correct. That is because of the report
narrated by al-Bukhaari (5105) from ‘Uqbah ibn al-Haarith (may Allah be
pleased with him) who said: I married a woman, then a black woman came to us
and said: I breast-fed both of you. I went to the Prophet (blessings and
peace of Allah be upon him) and said: I married So and so, daughter of So
and so, and a black woman came to us and said to me: I breast-fed both of
you, and she is lying. He turned away from me, so I came to him from the
other side and said: She is lying. He said: “How could you keep her as your
wife when that woman has said that she breast-fed you both? Leave her (i.e.,
divorce your wife).” 

For more information on the two issues mentioned above,
please see the answers to questions no. 804 and
175250 

However we should point out here that what is mentioned in
the question about the Shaafa‘i madhhab stipulating that there should be
four witnesses is not quite correct. Rather the view of the Shaafa‘i madhhab
concerning that is that breast-feeding may be proven by the testimony of two
men, or a man and two women, or four women. 

See: Nihaayat al-Muhtaaj (7/185) and al-Mawsoo‘ah
al-Fiqhiyyah al-Kuwaitiyyah (22/254). 

Secondly: 

The scholars (may Allah have mercy on them) have stated that
the ordinary Muslim does not have the right to choose from the madhhabs and
opinions of the scholars on the basis of his whims and desires by way of
finding a concession. 

Ash-Shaatibi (may Allah have mercy on him) said: The ordinary
Muslim does not have the right to choose between different views on a
particular issue, such as if the scholars differed and there are two
opinions, and the ordinary Muslim comes across these two views. Some people
may think that the two opinions are the matter of choice, as is the case
with regard to expiation, so he follows his whims and desires and that which
suits his aims, and leaves that which does not suit him; perhaps he may
quote, to support his choice, the view of some later muftis, and strengthen
it by quoting the words narrated from the Prophet (blessings and peace of
Allah be upon him): “My Companions are like the stars,” which we have
already discussed. Even if we assume that it is saheeh, it should be
understood as referring to a case where an ordinary Muslim went to a Sahaabi
or someone else to ask a question, and followed whatever fatwa he gave him,
whether it suits him or not. But if he comes across two different views,
then we should say: This case is not what the hadeeth is referring to,
because each of the two scholars (who have these different views) is basing
his view on evidence that led him to reach a conclusion different than that
to which the evidence of other scholars led, so these two scholars have
opposing views. Therefore following one of them on the basis of whims and
desires is in fact following whims and desires. We have discussed the
seriousness of this approach before. There is no option but to weigh up who
has the greater knowledge, and other similar ways. Moreover, the views of
these two scholars, for the common folk, are like two proofs for the
scholars; just as it is obligatory for the scholars to weigh up two proofs
and determine that one is stronger than the other, it is obligatory for the
ordinary Muslim to do likewise. If it were permissible to base one’s choice
on whims and desires in such matters, then it would be permissible for the
judge to do likewise, but that is not allowed, according to scholarly
consensus. 

Moreover, in issues where there are different views, there is
a Qur’anic guideline stating that one should not follow whims and desires at
all. Allah, may He be exalted, says (interpretation of the meaning):
“(And) if you differ in anything amongst yourselves, refer it to Allah and
His Messenger” [an-Nisa’ 4:59]. There may be two different scholarly
views concerning the issue facing the ordinary Muslim, in which case it must
be referred to Allah and His Messenger. That means referring to the shar‘i
evidence, which is furthest removed from following whims and desires,
because choosing one of the two views on the basis of whims and desires is
contrary to the concept of referring to Allah and His Messenger. 

Moreover, that approach will lead to following the easier
option among the differing views, without basing that choice on shar‘i
evidence. Ibn Hazm narrated that there was scholarly consensus that this is
an evil and is not permissible.

End quote from al-Muwaafaqaat, 5/79-82 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said: If it were permissible for the ordinary Muslim to follow whoever he
wants, then what is indicated by the words of our companions and others is
that it is not permissible for him to seek out the easier option at all.

End quote from al-Mustadrak ‘ala Majmoo‘ Fataawa Shaykh
al-Islam, 2/258 

Based on that, if you were both breast-fed from that woman,
and there were five or more breast-feedings, then you are siblings through
breast-feeding and it is not permissible for you to marry. In order to prove
whether that breast-feeding occurred, the testimony of one woman is
sufficient, according to the more correct opinion. 

With regard to what you mentioned about your love for the
girl, that does not give you the right to transgress the sacred limits of
Allah or to seek out concessions or the easiest scholarly view. Rather doing
so will divert you from seeking the halaal option. Other women will have
what she has, so do not let the Shaytaan mislead you. 

And Allah knows best.