Realistically, if Russia cannot annex the whole country, and cannot install a puppet regime over the whole country, then it's only option is partitioning Ukraine into Western Ukraine and Eastern Ukraine along the Dniper river, with pro-West independent Western Ukraine as a "failed state" (landlocked, ungovernable, economically backwards) and pro-Russia independent Eastern Ukraine as a puppet state like Afghanistan gov't. That's the only way to guarantee NATO/EU won't touch Ukraine with a 10-foot barge poll and still have defacto long-term veto over Ukraine's foreign policy, and still secure a sizeable buffer zone for Russia proper. That would be well worth the 15% hit in GDP to Russian economy, not some paper promise on neutrality.
In past, Putin has indicated they will "withdraw troops from border and have no intention for war" so may this is psychological warfare in negotiations. Don't blindly take Russia's words at face value.