Da'eef (weak) hadeeths, Dought & clear, - * Accumulate Voluntary Fasts and Make Them up in Sha‘ban?

How sound is the following hadeeth (narration): "The Messenger of
Allah (peace and blessings of Allah be upon him) used to fast three
days every month, and sometimes he would delay that until a year's
fasts accumulated and he would fast Sha'ban?"
Praise be to Allah
This hadeeth was narrated from the Mother of the Believers, 'Aishah
(may Allah be pleased with her) who said:
The Messenger of Allah (peace and blessings of Allah be upon him) used
to fast three days every month, and sometimes he would delay that
until a year's fasts accumulated and sometimes he would delay it until
he fasted Sha'ban.
This was narrated by at-Tabarani in al-Mu'jam al-Awsat, 2/320. He
said: Ahmad told us: 'Ali ibn Harb al-Jandeesaboori told me: Sulayman
ibn Abi Hawthah told us: 'Amr ibn Abi Qays told us: from Muhammad ibn
'Abd ar-Rahman ibn Abi Layla, from his brother 'Eesa, from his father
'Abd ar-Rahman, from 'Aishah (may Allah be pleased with her) who said.
Then he quoted it, then he said: This hadeeth was not narrated from
'Abd ar-Rahman ibn Abi Layla except via this isnad (chain of
narrators), and 'Amr is the only one who narrated it. End quote.
This is a da'eef (weak) isnad because of Muhammad ibn 'Abd ar-Rahman
ibn Abi Layla – the famous faqeeh (jurist) – of whom Imam Ahmad said:
He had a poor memory and his narration of hadeeth was inconsistent.
Shu'bah said: I have never seen anyone with a poorer memory than Ibn
Abi Layla. 'Ali ibn al-Madeeni said: He had a poor memory and his
hadeeth is weak. Hence the scholars classed this hadeeth of his as
da'eef.
Al-Haythami (may Allah have mercy on him) said:
Its isnad includes Muhammad ibn Abi Layla and there are some
reservations concerning him. End quote.
Majma' az-Zawaid, 3/195
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said:
Ibn Abi Layla is da'eef. The hadeeth of the door and the one that
follows it are indicative of the weakness of what he narrated. End
quote.
Fath al-Bari, 4/252
Ash-Shawkani (may Allah have mercy on him) said:
Its isnad includes Ibn Abi Layla, who is da'eef. End quote.
Nayl al-Awtar, 4/332
The scholars differed concerning the reason why the Prophet (peace and
blessings of Allah be upon him) fasted most of Sha'ban and there are
many opinions, including the one mentioned above, but the evidence for
it is not sound. Perhaps the first one who narrated it was Ibn Battal
in his commentary on Saheeh al-Bukhari (4/115). He also mentioned
other opinions that were narrated by al-Hafiz Ibn Hajar (may Allah
have mercy on him) and added to them, then he said:
More appropriate than this is what is mentioned in a hadeeth that is
sounder than what has been mentioned above. It was narrated by
an-Nasai and Abu Dawood, and classed as saheeh (authentic) by Ibn
Khuzaymah, from Usamah ibn Zayd (may Allah be pleased with him) who
said: I said: O Messenger of Allah, I have not seen you fasting in any
month more than you do in Sha'ban. He said: "That is a month that
people are heedless of, between Rajab and Ramadan, but it is a month
in which deeds are taken up to the Lord of the Worlds, and I like my
deeds to be taken up when I am fasting."
End quote from Fath al-Bari, 4/215
And Allah knows best.

Da'eef (weak) hadeeths, Dought & clear, - * Should he fast on the fifteenth of Sha’baan even if the hadeeth is da'eef?

Is it permissible, even after finding out that a hadeeth is da'eef
(weak), to follow it, by way of doing righteous deeds? The hadeeth
says: "When it is halfway through Sha'baan, spend that night in prayer
and fast on that day." The fast is observed as a voluntary act of
devotion to Allaah, as is spending that night in prayer (qiyaam
al-layl).
Praise be to Allaah.
Firstly:
What is narrated concerning the virtue of praying, fasting and
worshipping on the fifteenth of Sha'baan (al-nusf min Sha'baan) does
not come under the heading of da'eef (weak), rather it comes under the
heading of mawdoo' (fabricated) and baatil (false). So it is not
permissible to follow it or to act upon it, whether that is in doing
righteous deeds or otherwise.
A number of scholars ruled that the reports concerning that were
false, such as Ibn al-Jawzi in his bookal-Mawdoo'aat, 2/440-445; Ibn
Qayyim al-Jawziyyah inal-Manaar al-Muneef, no. 174- 177; Abu Shaamah
al-Shaafa'i inal-Baa'ith 'ala Inkaar al-Bida' wa'l-Hawaadith, 124-137;
al-'Iraaqi inTakhreej Ihyaa' 'Uloom il-Deen, no. 582. Shaykh al-Islam
[Ibn Taymiyah] narrated that there was consensus on the fact that they
are false, inMajmoo' al-Fataawa, 28/138.
Shaykh Ibn Baaz (may Allaah have mercy on him) said inHukm al-Ihtifaal
bi Laylat al-Nusf min Sha'baan(Ruling on celebrating the fifteenth of
Sha'baan):
Celebrating the night of the fifteenth of Sha'baan (Laylat al-Nusf min
Sha'baan) by praying etc, or singling out this day for fasting, is a
reprehensible bid'ah (innovation) according to the majority of
scholars, and there is no basis for this in sharee'ah.
And he (may Allaah have mercy on him) said:
There is no saheeh hadeeth concerning the night of the fifteenth of
Sha'baan (Laylat al-Nusf min Sha'baan). All the ahaadeeth that have
been narrated concerning that are mawdoo' (fabricated) and da'eef
(weak), and have no basis. There is nothing special about this night,
and no recitation of Qur'aan or prayer, whether alone or in
congregation, is specified for this night. What some of the scholars
have said about it being special is a weak opinion. It is not
permissible to single it out for any special actions. This is the
correct view. And Allaah is the Source of strength.
Fataawa Islamiyyah, 4/511.
See also question no. 8907.
Secondly:
Even if we assume that the hadeeth is da'eef (weak) and not mawdoo'
(fabricated), the correct scholarly view is that weak ahaadeeth should
not be followed at all, even if they speak of righteous deeds or of
targheeb and tarheeb (promises and warnings). The saheeh reports are
sufficient and the Muslim has no need to follow the da'eef reports.
Nothing is known in Islam to suggest that this night or day is
special, either from the Prophet (peace and blessings of Allaah be
upon him) or from his companions.
The scholar Ahmad Shaakir said: There is no difference between rulings
or righteous deeds; we do not take any of them from da'eef reports,
rather no one hast he right to use any report as evidence unless it is
proven to be soundly narrated from the Messenger of Allaah (peace and
blessings of Allaah be upon him) in a saheeh or hasan hadeeth.
Al-Baa'ith al-Hatheeth, 1/278.
And Allaah knows best.