Both myths are false. 

Triathlon is an aerobic sport, meaning that the duration and intensity of the event requires primary oxidative energy to perform the event. Trainng your oxidative energy systems maximally should be the goal of your training.  Decreasing training volume to "endurance" effort only will actually decondition you a bit from your race fitness. Why waste that time and energy when it's not needed?  Instead, spend the early "base training" season working on improving your lactate threshold in all 3 sports.

WHy? Studies have show that lactate threshold is the single biggest predictor of performance in endurance events.

When doing "long slow rides" commonly referred to as "Zone 2" or "endurance" training, you are working well below your threshold. Long base rides DO provide some training benefit in terms of improving VO2 max (hte maximal amount of oxygen you can utilize while exercising), but there are much more efficient (faster) ways to do this.

Why not spend your time maximiizing yoru return on investment as far as  allowing your training to improve your oxidative engines?

As far as doing intensity at the beginning or at the end of a training session, it depends what type of intensity...anything aerobic (threshold or VO2 max intervals) can be done at any time during the training. Anaerobic intervals should be done at the beginning of a session after a warmup in order to maximize your training response from them (if you are fresher, you'll produce more power).  But if the goal is to mimic race efforts, doing anaeribic sessions after a longer endurance warmup may better simulate the demands made later in a race.  This is more applicable to cycling than triathlon, but if you have a hilly finish on the run course esepcially, why not throw in a few short VO2 hill repeats (3 minutes) at the end of a training brick once a week? After all, that's what you'll be doing in a race.