a) Custody of children: Co spouses would be able to argue for visitation rights depending on involvment, using an existing UK House of Lords precedent, where the lesbian partner of the mother was able to argue that she had a substantial role in the childs upbringing. At the Appeal stage, she even won Residency, which was only overturned in the HoL.
b) Yes, but see following answer
c) There is a right to not testify, however if they have the goods on one gang member there is nothing preventing plea bargaining for testimony. It is a right to not testify against your partner, but you are not prevented from doing so.
d) In the UK, whilst polygamy is illegal, polygamous relationships lawfully established in other countries are recognised. Wives are entitled to benefits as single wives. (recent change in law). In general UK taxation no longer has marriage advantages built in, explaining why less people getting married.
e)Yes, where such pensions are tranferrable to the spouses.
f) Yes you could, but proof would be required that a marriage was not a sham and survived for a substantial time.
g) Pensions. The 10 way split thing is again irrelevant. The state provides a basic pension to each widow, any further pensions are a matter of private arrangements.

Hypotheticals
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a) all partners in a marriage must give consent for other members to join the marriage; that eliminates hypothetical (1) as W1 has not consented for W2 to join the marriage. M must divorce W1 to marry W2 unless W1 consents.
b) The answer involves swimsuits and mud wrestling. Winner decides.
c) The man has an economic value to the marriage which must be compensated for. Punitive damages are a separate matter.

Also, one notes in passing that you assume 1 man and lots of women. There may be cases with the reverse.

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Girls come and go but a mortgage is for 25 years -- JtL