There is no way to know how many accounts one investor might have.
The danger of 51% attacks is very real until and unless the coin becomes much more widely distributed and used for more things besides investing in the JD bankroll. Most people trust dooglus (or the coin would have no value) but things can go wrong. The site can be hacked. He might sell it. Or, he might turn bad after all.
More large diggers are probably an advantage for security especially if they stake their own coins. If they invest in JD or sell to people who invest in JD then it is no change. It's pretty unlikely that a single new (huge!) digger is going to come along and dig up enough to 51% attack by himself and that becomes even more unlikely the more coins are dug up (including by this current whale digger) and staked.
I agree with you on this point, as much as I think dooglus is an honorable person you should never keep all your clams in one clam basket. I keep a small portion on his site, but a majority of what I have stakes in multiple wallets. More people staking and less clams in a centralized location makes for a better network. We could solve the issue now if people would just take half of what they have on Jd and stake on a local wallet.