You could solve the repick "bug" by reducing the level of the passive buffs on Argonax by the level of the repicking hero before removing the actual hero. Remember that unlike researches, abilities can be reduced in level.
Heroes of leavers still are controllable, which means it shouldn't be a problem.
If you think it is because you have to micro 2 heroes, well you'll always have that problem, whether Argonax "levels" or not.
Additionally, you could lock the gates for the first 10 minutes to avoid summonspams early game.
Alternativelly you make Argonax 3/4th as strong as he is now, making him technically impossible at low levels, and make the passive leveling abilities less strong but still effective enough to make him stronger over time.
Actually, you might as well change the ability bonusses depending on the level: as an example I use some numbers pulled out of my ass, but ofcourse it's up to you.
Level 1: 250 additional HP (making him much stronger at lower lvl)
Level 2: 325 HP (+75)
Level 3: 400 HP (+75)
Level 4: 450 HP (+50)
Level 10: 750 HP (+50)
Level 11: 775 HP (+25)
Basically it means that the bonusses are lower the higher the levels of your heroes, while at the same time it does make him stronger... a lvl 12 Argonax would be double as strong as a lvl 4 Argonax (in a 1v1 you're both around lvl 2), while it'd take an additional 16 levels (lvl 28 Argonax - which means in a 1v1 your heroes will be around lvl 14) to become 4 times as strong than a lvl 4 Argonax...
Balancing testing will tell you what numbers to use on the abilities...
But then again, it's only a suggestion
