how accurate is the network adjusted time.
Not at all accurate by comparison with other network protocols designed to produce accurate time. This is because it didn’t need to be very accurate.
As far as I know, Network Adjusted Time is something like the sum of local time plus the mean of the difference between local time and the reported time of each peer node whose time is within 70 minutes of local time. I think it is evident that this isn’t always going to be very accurate. When using that, I would assume it can be tens of minutes out.
By way of contrast, Network Time Protocol (NTP) can produce time accurate to milliseconds.
There have been changes that affect whether network adjusted time is used at all. See references below.
two consecutive blocks of Bitcoin might show timestamps that show a difference much greater than two hours plus 15 mins
Yes.
However, the timestamp difference of two consecutive blocks is not particularly important.
It is the mean of timestamps of eleven consecutive blocks that is important.
See