Personally, I would keep to your commitment with your original buyers, at least for now. You have agreed the deal with them, and over the coming days you will see how serious they are, by how quickly they start instructing solicitors, getting surveys and mortgage offer in place etc.
Hopefully your estate agents came clean that your house was "under offer" at that time .... hmm maybe not. Anyway, if they have held back on the facts, that's just how they work.
The response I would give to the people offering you more money is that, on principle you will be honouring the original agreement, but if they would like to have first refusal with a timelimit of x weeks, then depending on how much they have fallen in love with your house (and its uniqueness), you may find they hover for a while to see how things go.
They will either choose to keep you on their "wanted" list and keep looking elsewhere - or else they will walk away.... meanwhile your first purchasers will feel confident and move swiftly on, so you can get on with your lives (house buying does take years off your life 
)
Don't forget there is nothing to stop those vendors taking you all the way to exchange on a false promise, then suddenly drop their price back to the original, or lower - you are caught over a barrel by then, back to square one (and feeling awful!). People play some vile tricks, its all legal, but not ethical!
If your original buyers start to look flakey in the coming days / weeks or you don't feel they aren't financially viable (and there is a statistical probability of that happening), then there is no reason your estate agent can't offer your second buyers the option, based on the fact your first buyers are taking far too long.
I personally wouldnt get into the gazzumping situation, its grubby and not naice, and yup people do it all the time, which over-inflates house prices and may well put people in debt because they become so fixated on a house they will pay any price (even though it puts them in hock).
You will keep the moral high-ground. I totally agree with Karma in house-buying - one day you may be praying for it not to happen to you! The estate agents rarely set the conditions for morality, sorry to say, you need to decide what your own boundaries are, and what you can live with.