Let me answer your last question first.
That 100 hours is 100 hours of actual processing time, so two tasks, each with 100 hours to go, will take a total of 200 hours before they are both completed. But Boinc's task switching may mean that one of them will finish long before the other. (There are too many factors that could influence the switching for me to even pretend I know it all!)
To address your concern, that 270 hours may not be an accurate estimate. Since this is possibly the first Work Unit (WU) you're crunching from that project it could even be a very inaccurate estimate! Boinc uses a Duration Correction Factor (DCF) to correct for wrong estimates. If a WU takes longer than was estimated (by the project, by the way), then the factor is adjusted upwards to fully compensate for the difference. When you first add a project the DCF is 100. As an example. if a WU starts out expecting to take 3 hours and actually takes 6 then the DCF for that project will be changed to 200. Note, however, that reducing the DCF is done much slower to avoid an unexpectedly short WU causing a too low DCF. In other words, if it expects 6 hours and takes only 3 hours, the DCF would not go from say, 100 to 50, but perhaps to only 90 (I don't know the exact amount it drops by, but I seem to remember reading about 10%). This avoids having more WUs downloaded than can be completed in the required time.
Since this all affects the estimated times, I hope you see why I answered the last question first!
To answer the first question. If the DCF of the project in question is too low, it will be adjusted, but only after you let the WU finish. What you may like to do is after a day or so, see if the remaining time has come down enough that you might finish in time.
Wow, I thought I was going to 'quickly' reply to your posting!
When Boinc is left alone for a long period it eventually seems to sort it self out, but you may miss a deadline or two while it does so

Not so bad for a 1 hour WU, but for 270 hours...