This reminds me of The Mythical Man Month article:
"He had added more programmers to a project falling behind schedule, a decision that he would later counter-intuitively conclude to have delayed the project even further"
be careful about throwing more devs at it.. I would say a project manager.. or better yet, putting a person between you and the stakeholders will be better for you and for them..
It's nice of you to take on this yourself, but the "manager" / "stakeholder" or "person who cares about timeliness" shouldn't be speaking to you directly. At least ideally. for one, you dont want to deal with their pressure and their reactions to your honest answers..
they ask "can it be done in a week" you want to say "no way in H*LL", but because they're a manager / boss / important person... you say "yes", and in your head you mean.. "yes, if i dont sleep"...
so it's good if you can get someone between you and the person asking.. that way they hopefully wont shoot the messenger (if they do messengers are cheap!! lol).. and you can be honest and not stress over timelines that are being set without the input of a dev..
Also, getting someone in there to line item what needs to be done will allow you to estimate what each item will take to get done.. then the stakeholder can choose what to include and will have reasonable expections as to when they can expect the tasks to be done, as well as the entire project..
And then make sure to pad your hours to make sure you either meet them, or come in under the estimate.. I always tell anyone who asks me for a timeline to multiply it by three to be safe, even if i'm very convincing about it.. because i usually include a fair amount of "hope" in my estimates and am overly optimistic
UPDATE: As far as Paired Programming (LINK).. I would say this could be a good idea. For one, it'll give you some company, also, someday, they could end up with 2 programmers attacking the project eventually increasing productivity, but that wouldn't happen until the other programmer is fully up to speed..
I'm not sure they would need to hire a new full time project manager.. maybe someone already in the company might have some extra time to do some light "task management" and be able to be the liaison between you and the person asking about timelines, that way they could get you some help (another dev) AND get a buffer between the dev and the stakeholder.. the danger of having a project manager whose only job is to bang on you could backfire and become just as annoying as the current situation..