except if you're living in a community property state, a multi-member LLC must file a partnership return. if you don't meet this exception you are facing substantial penalties for filing late, but if this is the first time so filing, the IRS can waive them.
it is not clear as to which spouse you were married to on 12/31/2021, if either, but I am going to assume it's the current one. why do you want to use MFS? that will probably cost you more in taxes because certain credits are not allowed and both must use the same method - either the standard deduction or itemized deductions. if you are not the custodial parent (the one your daughter lived with for most of the year (nights) you need a signed and dated form 8332 from your ex to claim her. it must be filed every year you claim her and must be attached to the return since you can no longer e-file for 2021.
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as to how you want to split the partnership (LLC) income/loss. the IRS regulations specify that the allocations must have a substantial economic effect and substantiality under Regs. Sec. 1.704-1.
Economic effect is satisfied based on a three-part test: (1) the partnership must maintain capital accounts in accordance with Regs. Sec. 1.704(b)(2)(iv); (2) liquidating distributions must be from positive Sec. 704(b) capital accounts; and (3) the partnership must contain a deficit restoration obligation (DRO). As a DRO requires a partner with a negative Sec. 704(b) capital account to contribute more cash to the partnership upon liquidation, a newly formed partnership with a DRO is exceedingly uncommon. Fortunately, Regs. Sec. 1.704-1(b)(2)(ii)(d) allows an allocation to have economic effect without a DRO, provided the partnership agreement contains a qualified income offset (QIO). A QIO allows partners to receive certain allocations that cause their Sec. 704(b) capital account to "unexpectedly" go negative, so long as they are obligated to be allocated income in the future to restore their Sec. 704(b) capital account at least to zero "as quickly as possible."
The second part of the two-part test for substantial economic performance is substantiality. For an allocation to be substantial, there must be "a reasonable possibility that the allocation (or allocations) will affect substantially the dollar amounts to be received by the partners from the partnership, independent of tax consequences" (Regs. Sec. 1.704-1(b)(2)(iii)(a)). This section largely prevents a partnership from manipulating timing differences with respect to deductions or manipulating tax rates by allocations of class of income, such as all capital gain to a partner with large unused capital losses, among others.
