Alright, in that case I say drop your Constitution 1 point and add it to Dexterity instead. Then after that, you can leave Strength and Dex at 13 each and qualify for Heavy Blade Mastery at 21st. The big advantage is that you're going to get 4 more level-up points for Wisdom and/or Constitution, and all three of your Wisdom, Constitution, and Charisma will end up even, so you'll be getting the most out of your ability scores.
Heavy Blade Opportunity does look nice, but the main draw of it is for Paladins is to use Charisma instead of Strength for Opportunity Attacks, and even at 20th level your Charisma modifier will only be 3 points higher. With Blade Opportunist, your Opportunity Attacks will only be 1 point behind your standard attacks at the most.
That goes a bit against what I usually recommend, but trying to shoehorn a low-level Paladin into a Leader role is not going to work most of the time. I strongly recommend taking some Multiclass feats to get some Warlord powers and maybe also the Raven Queen's Blessing feat. Student of Battle will give you what essentially amounts to another use of Lay on Hands a day, and Acolyte Power will get you Inspiring Reaction initially and Warlord's Banner later on, both of which are impressive Healing powers in their own right, but more importantly they're Encounter powers. That said, don't do Paragon Multiclassing. You're going to want the Hospitaler Path.
If there is no Cleric or Warlord in the party, then I think you should probably either get one of the other players to roll one or roll one up yourself. Having a Leader in the party really is a great help. Also, if you are the only Defender in the party then devoting a lot of resources to Healing may detract from your ability to actually fill the Defender role. Regardless, I gotta say it again: there really should be a Cleric or Warlord in the party.