Moving carry from 90 days to 150 costs a shell deal roughly 5k of offer room

You are missing a cost on the 150 day path. Most 12 month rehab notes are written short, and plenty of them are 6 or 9 months with an extension option that costs a point or a fixed fee. If the job runs to 150 days of construction, day 150 is when you list, and a 30 to 45 day escrow puts the sale at month seven. So the 5,200 has an extension charge sitting behind it, maybe 1,800 on a 180k note, and that fee is charged whether or not you needed the whole extension. Confirm the extension terms in writing before you model anything, because they vary a lot between lenders. On your actual question, the reserve is not a substitute for the bid. A reserve pays for the overrun. It does not make the overrun free.

The 60 days is almost never spread evenly across the job. It's usually one permit sitting on a desk, or one trade that won't come back. If you're reconfiguring layout you're waiting on plan review, and that clock isn't yours.

Most good crews book out six to eight weeks before anything goes wrong, so an investor who expects a start the week of closing should ask the contractor what the calendar actually looks like. On the 143 versus 165 gap, stop treating the 92k as fixed. Ask the contractor what sits in it as an allowance rather than a bid, because on a gut the tile, cabinet and lighting numbers are usually placeholders, and if two of those are light by 4k each then the 95k carrying figure was the optimistic one and the carry argument is the smaller of your two problems.