试着回答
1. 可以在public scheme下购买二手组屋
http://www.hdb.gov.sg/fi10/fi10321p.nsf/w/BuyResaleFlatPublicScheme?OpenDocument
2. 如果该组屋是她第一套房产，CPF OA可以全部使用，但是要注意一下该女士到55岁时候公积金账户里必须要有minimum sum.
http://mycpf.cpf.gov.sg/CPF/my-Cpf/reach-55/Reach55-2.htm
买房其实不是问题，问题在于两年后大女儿成年，可以继承父亲的公寓的时候。
HDB有规定说不管是Owner还是occupier，只要是在购买HDB申请上列名的人，都不可以在MOP期间购买私宅。但是，这个私宅范围包括：
* includes but not limited to:
HUDC flats (Privatised and Non-Privatised)
Properties acquired by gift
Properties inherited as beneficiaries under a will or as a result of the Intestate Succession Act
Private properties
Properties owned/acquired/disposed through nominees regardless of properties location
后面又有个：
Exemption to Exceptional Cases
HDB may consider granting exemption to exceptional cases based on the merits of each case. If you wish to seek exemption to buy a flat direct from HDB, please complete the property questionnaire and submit it to HDB.
http://www.hdb.gov.sg/fi10/fi10321p.nsf/w/BuyingNewFlatEligibilitytobuynewHDBflat?OpenDocument#PrivateProperty
结论就是最好去跟HDB面谈这个私宅继承的问题。