You can't decrease your character's current level (or, rather, the experience level), but you can decrease skill levels and the amount of experience in them.
AdvSkill <skill> <nn>
Advance the specified skill.
AdvSkill registers the increase and will count towards your next levelup. The number represents the experience level to add to the
player.advskill destruction 1 will only add 1
experience towards the next skill increase, it will not advance
Destruction 1 skill level). Some skills require far more experience
points than other skills, crafting skills level quickly with few
points, but armor skills require much higher numbers.
And from this page:
[Before the 1.9 patch], the following command would set the exact
experience you would have in your Smithing skill to 0, also making the
Smithing level 0:
player.setav SmithingSkillAdvance 0
The following command would set the exact experience you would have in
your Smithing skill to 3746, also making the Smithing level 12:
player.setav SmithingSkillAdvance 3746
These values are absolute, meaning you can readjust your skill levels to approximately reflect your lowered level.
With [after?] the 1.9 patch, you can still achieve similar effects by
using the advskill command together with the setav command, but
it's a much less exact process. For example, to set your skill
progress for the current level to 0, you would use setav to decrease
the relevant skill by one, then use advskill in decreasing amounts
to slowly advance your skill progress towards the next level until the
skill goes up by one. Similarly, you can very roughly figure out how
much skill XP a specific action is worth by saving your game, using
the skill (or casting the spell) for a specific duration or a specific
number of times, then noting how much progress you've made in the
skill. After that, you would reload and use the advskill command to
determine how many skill points it takes to reach the same amount of
I think the better (saver) choice in your situation would be to opt for any of the following circumventions:
Loading the mod 'Experience' after the startup quests have stopped;
Trying to find (or create) a patch to counter these effects;
Seeing if you can edit the amount of experience points gained from those startup quests.