Are there any cornerstone investors, sponsor, accredited investor, institutional investors who have taken up the P2P loans? If no, why not? ?

I say yes.
Accredited investors are found in p2p platforms and some platforms provide bonus for these investors.

Sponsors maybe the Angel investors/ venture capital who give them relevant series fundings (with objective)

Insitutional investors: There are. I can't remember any example. I ask them about it during the fintech festival. Maybe DBS??? DBS has collaboration with funding societies.

There are in the US (lending club for example takes on a number of transactions on to its own books), and we will definitely see this taking place more prominently in Asia with the advent of debt focussed funds.

Given the current state and relatively new creation of p2p platforms in asia though, the relative size of P2P market vs size of debt funds which tends to be a lot larger is causing the slow uptake.