In another MMO they had a system going so say you killing this mob who has say 1K hp. Someone comes along and thinks yeah I'll have a big of the action. The mobs HP rose to 2K to accommodate for this new person. Thus allowing everyone to get a fair chance of killing the mob. SE could have been more flexible in how they achieved this knowing the influx of people who would be wanting to do it.

It would make the fights more interesting rather than the 10 second zerg we currently have.

As to the marks that are fate bosses it does seem a bit ridiculous considering spawn times and amount of fate related objectives we've had to deal with recently but if you don't really want to do them you can always discard the mark bills and wait for fresh ones the following day.