The whole thing is perplexing, and I don't think we can look at individual positions separately anymore. As you say, each decision could be base don multiple things. But the overall decision to raise and maintain so much cash must have some reason behind it. There are a few possible reasons:
1. Could be that he wants massive cash available to take advantage of good bargains when the SHTF (debt crisis, CRE crisis, government paralysis crisis, whatever it may be). This is an entirely reasonable supposition.
2. But it could also be a liquidation event on the horizon someday that is being prepared for. Maybe upon WEBs death, all his shares will be liquidated and instead the cash used for the various foundations (I know everything says otherwise, but perhaps it is still planned so that the foundations aren't made reliant on a non-WEB manager). That would make $300B+ logical, $100-150B to buy all of WEBs shares, plus another $150-200B for the new guys to use to tun the business, to invest in their own picks.