It's not like I have a lot of experience in this (and the UK may be different), but:
> what's the shortest transition-the-product-to-new-ownership period I can reasonably look for?
Whatever you can get away with, but a small number of years is what I'd call "normal".
> Is there typically any short-term payout for acquired employees or will everything be in salary and slow-vesting options?
The latter - the whole point is to try to get you to stick around, after all.
You can expect to get a good price for your (non-option) equity immediately though.
> Is there any likelihood of an acquirer being interested in purchasing the product but not its developers?
If the product is profitable, then yes, but they will certainly want the developers for a transition period.
> Assuming I do stick it out as an acquihire, what's the likelihood I'd be allowed to remain in charge of my product vs having to execute on someone else's vision?
How good are you at office politics? There will be people above you, unless and until you can get yourself promoted above them. Your working relationship with your boss is what you and they make it. By all means try to find and sound out the person who's going to be in charge of you, but realistically the acquirers probably don't know themselves who'll be running your division in 3 months' time.
> When, if ever, is it appropriate for me to ask to be included in negotiations with acquirers? Most importantly: how do I best stand up for my own interests here, without being the asshole that ruins the deal for everyone?
Honestly, any deal is going to be about the (monetary) interests of equity holders. The current owners have nothing to gain from treating the employees well and will not negotiate particularly well for the employees' interests (and why should they?). The acquirers will give the employees whatever terms they think are in their own best interests - which may include some form of slow-vesting golden handcuffs, but that will be determined solely by how much they (the acquirer) want you to stick around.
As an employee you don't even get a seat at the table - you get what's in your contract with your current company, plus whatever you personally negotiate with the acquirers. By all means negotiate that, but that's for after the sale is agreed, and treat it as what it is: an employment negotiation with an entirely new company. Remember you can always walk away.
As a shareholder, on the other hand, the board should be representing your interests as a shareholder. But that probably boils down to selling the company for the highest price possible, which is what they'll be aiming for already. So I don't see any real value in inserting yourself into that negotiation.
> what's the shortest transition-the-product-to-new-ownership period I can reasonably look for?
Whatever you can get away with, but a small number of years is what I'd call "normal".
> Is there typically any short-term payout for acquired employees or will everything be in salary and slow-vesting options?
The latter - the whole point is to try to get you to stick around, after all.
You can expect to get a good price for your (non-option) equity immediately though.
> Is there any likelihood of an acquirer being interested in purchasing the product but not its developers?
If the product is profitable, then yes, but they will certainly want the developers for a transition period.
> Assuming I do stick it out as an acquihire, what's the likelihood I'd be allowed to remain in charge of my product vs having to execute on someone else's vision?
How good are you at office politics? There will be people above you, unless and until you can get yourself promoted above them. Your working relationship with your boss is what you and they make it. By all means try to find and sound out the person who's going to be in charge of you, but realistically the acquirers probably don't know themselves who'll be running your division in 3 months' time.
> When, if ever, is it appropriate for me to ask to be included in negotiations with acquirers? Most importantly: how do I best stand up for my own interests here, without being the asshole that ruins the deal for everyone?
Honestly, any deal is going to be about the (monetary) interests of equity holders. The current owners have nothing to gain from treating the employees well and will not negotiate particularly well for the employees' interests (and why should they?). The acquirers will give the employees whatever terms they think are in their own best interests - which may include some form of slow-vesting golden handcuffs, but that will be determined solely by how much they (the acquirer) want you to stick around.
As an employee you don't even get a seat at the table - you get what's in your contract with your current company, plus whatever you personally negotiate with the acquirers. By all means negotiate that, but that's for after the sale is agreed, and treat it as what it is: an employment negotiation with an entirely new company. Remember you can always walk away.
As a shareholder, on the other hand, the board should be representing your interests as a shareholder. But that probably boils down to selling the company for the highest price possible, which is what they'll be aiming for already. So I don't see any real value in inserting yourself into that negotiation.