Rather than redevelop the game, just stay in the heroic tier.
But re-evaluate the "10 encounters per level" guideline. Instead of having 30 levels with 10 encounters each (totaling 300 encounters) have 10 levels with 30 encounters each (totaling 300 encounters).
If your player's can get over not leveling up after each session, there are a lot of subtle rewards to this style of gameplay. There are some tradeoffs too, but it can and has been done. In OD&D "Fighting-Men" were capped at level 9, "Lord", with 240,000XP, for example.
The thing is, more than any other edition, 4e isn't about the numbers that much. The actual stuff you can do changes from level to level, more than the numbers do.As you can see, I could add 5 more levels below lvl1, without having create a new mechanic for the monsters.
It's a start, and I'm very far from the finish line, but you gave me enough ideas, to be able to actually pull it off.
I'll echo the above poster who mentioned Basic D&D (or Labyrinth Lord, freely available online) - it's a lot closer to what you're looking for and a lot less work. It also avoids any confusion on the players part as to what they're playing if you're forming a new group.