Some of these issues are currently being worked on. Some have been able to have been resolved by clearing the cache and cookies on your browser. This must be done prior to opening up the developer.paypal.com site and most developers have better luck with this when using Firefox or Chrome. There appear to be more issues with IE, than the other browsers.
If doing this does not resolve your issue, you will want to open up a ticket with PayPal Merchant Technical Services. This way your issues can be tracked, and it will allow PayPal to gauge the number of merchants being effected by the different issues. Also, this will allow you to be notified once the issue is resolved.
As a workaround, you could also try using the API credentials that are within the sample code/SDK’s for testing purposes. This will allow you to test the API calls, though you just will not be able to log into the Merchant account and make changes to it. This may or may not work for you depending on your needs.