As you will recall, we saw a machlokes in the gemara between the Rabbis and Ben Teima as to whether terumah or tevel is more a serious איסור. Based on what we learned we can understand the root of their machlokes.
The Rabbis hold that terumah is more serious because they subscribe to Rashi's theory that tevel is forbidden because it is chullin and terumah mixed together. Therefore, we must say that eating terumah - which is pure איסור - is a more serious aveirah. That is what they mean when they say טבל קל משום שאפשר לתקוניה - This is another way of saying that tevel is less חמור because it is terumah mixed in with chullin and therefore by separating the terumah one may eat it. So tevel is just a mixture while terumah is pure איסור and unfixable [for a yisrael] - as the gemara says. The inescapable conclusion is that תרומה is more חמור.
And the fact that טבל is אסור לכל [Ben Teima's reason for saying that טבל is more חמור] doesn't bother the Rabbis because according to the them, since tevel can't be consumed by kohanim it must be that terumah is considered אכילה משלחן גבוה [as we explained earlier] and is thus also חמור and considered אסור לכל. So both טבל and תרומה are אסור לכל and תרומה can't be fixed while טבל can be fixed so תרומה is more חמור and a choleh should eat טבל.
Ben Teima follows the approach that we outlined according to Tosfos, namely that טבל is an איסור עצמי an independent איסור and not a mixture of terumah and chullin. Thus, we are not compelled to say that תרומה is משלחן גבוה. Since tevel may not be eaten for its own independent reason and not because of its terumah status we have no difficulty understanding why kohanim can't eat it. This difficulty forced us to say [according to Rashi] that tevel and terumah are like a korban before the zrikas ha-dam and after. But now we assume that tevel is independently אסור and have no problem explaining the איסור אכילה to kohanim. If so, תרומה is more קל because it is permitted to kohanim [as אכילה הדיוטית - regular eating and not from the Divine table] while טבל is more חמור because [as the gemara says] it is אסור לכל.
AHHHHHHHHHHH!!!!!