If it’s indeed as useful an acquisition to the buyer as it sounds, should you not be priced at higher than your last valuation - why are you priced lower?
This mis-pricing is what is causing investor B to lose.
Ideally you would sell at your last valuation plus some small increment so Investor B would get a “no-profit no-loss” deal with returns at least equivalent to the G-Sec rate, and later the earn-out would hopefully kick in for their reward in taking on the massive risk they took with you.
The earn-out is your reward for successful transition, handover and integration, it is in place as a risk-mitigation mechanism to avoid the firm dying out after acquisition.
Not saying this is the case here, but it’s very normal for companies to buy out smaller competitors to shut them down. A very famous example is the “face unlock” on Apple phones, which used to be a startup that used a grid of invisible infra-red laser dots to map a 3d object. This tech was licensed out, for example to the Kinect, and I even had a Lenovo laptop with this feature in the webcam, but once Apple bought them out they could no longer sell to anyone else, because Apple wanted to keep this feature exclusively for their phones only. So you never really know what happens post-acquisition.
Earn-outs are not part of the purchase price and should not be seen as such, much less sold to an existing investor in that way.
Simple Notification Service?