Pricing. Every price comes from discounting the full cash flows, not a duration estimate. Individual Bond buys one bond and holds it as it rolls toward maturity. Bond Fund keeps a constant maturity by rolling into a new par bond each month, so its income resets to whatever yields are doing, the way a fund's income does.

Curve rolldown. With rolldown on, each bond is priced off the current Treasury curve at its remaining maturity, and your rate path shifts the whole curve up or down in parallel. "Rates unchanged" then means the curve keeps today's shape, which is a scenario, not a forecast. When the curve slopes up, aging bonds roll into lower yields and pick up price. When it's inverted they roll into higher yields and lose it. With rolldown off, one yield is used throughout.

Reinvestment. Coupons are reinvested at the bond's own yield, the standard total return index convention.