How to create a diamond inheritance hierarchy when I cannot modify all of the base classes?

I am having a hard time with a dreaded diamond problem. For a reminder, here is the classical class hierarchy of this problem:

    B
   / \
 C1   C2
   \ /
    D

To solve it, the standard solution is to make C1 and C2 use virtual inheritance to inherit from B.

My problem is that B and C1 are from an SDK that I cannot modify. Example below where I cannot make SubClassB inherit virtually from Base. Classes: PureVirtualBase, Base and SubClassB are from the SDK I use. I cannot modify them. SubClassA and Leaf are my custom classes. I can change them.

     PureVirtualBase(SDK)
           |
        Base(SDK)
       /        \
 SubClassA   SubClassB(SDK)
       \        /
          Leaf

In such a situation where SubClassB cannot be changed to use virtual inheritance from Base. How what should so that:

• Leaf instance only contains one Base
• Avoid the ambiguity when trying to access functions defined pure virtual in PureVirtualBase and implemented in Base

class PureVirtualBase
{
public:
    PureVirtualBase()
    {
        cout<<"<<PureVirtualBase::PureVirtualBase" << endl;
        cout<<">>PureVirtualBase::PureVirtualBase" << endl;
    }
    virtual int f_PureVirtualBase()=0;
};
class Base : public PureVirtualBase
{
public:
    Base(std::string id) {
        cout<<"<<Base::Base:"<<id << endl;
        m_id=id;
        cout<<">>Base::Base:"<<m_id << endl;
    }
    virtual int f_PureVirtualBase() {
       cout<<"Base::f_PureVirtualBase" << endl;
       return 1;
    }
private:
    std::string m_id;
};
class SubClassA:  public virtual Base
{
public:
    SubClassA(): Base("From