Under the plan which has found increasing favour among SPL and Scottish League clubs, the two top divisions of 12 would split into three mini-leagues of eight after 22 games.
The top eight would then play for the title and European places, the middle eight would play to decide whether they started the next season among the leading 12 clubs, and the bottom eight would play to avoid relegation to the following season's third tier of 18.
Another meeting of a working group on league reconstruction will be held tomorrow – the third in a month – and a consensus is forming that the SPL's 12-12-18 proposal is preferable to the SFL's alternative of three divisions of 16-10-16.
The change could be in place for next season under a single, merged league body with a new model for income distribution, play-offs and a pyramid structure.
But supporters have frequently called for a top tier of at least 16 teams and those who feel they have been ignored are likely to be deeply resistant to leagues of 12 with a messy mid-season split.
"I think the major weakness in a top league of 16 is the lack of games," Longmuir told SportTimes, alluding to a campaign of only 30 league games.
"And the two 12s into three eights has a major weakness in that it is convoluted and complicated for fans to engage with. There is no perfect solution here. I think the biggest issue will be getting fans to engage with it. A league of 16 is easy to engage with, it's a straightforward and simple formula, 15 games at home and 15 away.
"We're looking at the merits of both and understanding the pitfalls of both, but we're putting that aside and recognising that there are four big things here that everyone in the room is agreed on: one league body, play-offs and pyramid, better distribution of wealth and a governance model that is not stymied by what is effectively a two-club veto.
"We would struggle to get a 16-team top league because of the lack of games but it could be possible to sell the other idea."
Longmuir dismissed suggestions that league reconstruction was being rushed through in order to minimise the amount of time Rangers spend below the top flight.
He said: "Neither proposal would 'fast-track' Rangers. A 16-10-16 would take them the same amount of time to get to the top league as they would in the current structure.
"With the 12-12-18, even if it was in place for next season, Rangers would not be in that top division."