Moving carry from 90 days to 150 killed a shell deal by roughly 5k

You're 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 your job runs to 150 days of construction, you're not selling on day 150, you're listing then, and a 30 to 45 day escrow puts you 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, 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 doesn't 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.

My crews book out six to eight weeks and that's before anything goes wrong, so when an investor asks me to start "as soon as you close" I tell them what my calendar actually looks like. On the 143 versus 165 gap, I'd stop treating the 92k as fixed. Ask your contractor what's in it as an allowance rather than a bid, because on a gut the tile, cabinets and lighting numbers are usually placeholders, and if two of those are light by 4k each your 95k carrying figure was the optimistic one and the carry argument is the smaller of your two problems.