If an inventor submits an idea and it has been accepted, and he makes no tangible device (it remains intellectual rather than carried out). But then the inventor does not pay the maintenance fees, does the invention become public domain, or is it possible to patent the idea again?
No, the idea is not patentable anymore. Once any person anywhere publishes knowledge on how to create the invention (whether publishes it in a patent, online, in a novel, etc...), the idea can no longer be patented. Clerks at patent offices have to review many sources (generally patent offices in other countries, existing patent applications, engineering journals, etc...) before they say there is no prior art.
If the inventor (or applicant in case the applicant is not the same as the inventor) fails to pay a maintenance fee, the U.S. Patent Office will allow the person six months before the patent application (or granted patent) is declared abandoned.
The amount of time after failure to pay and the declaration of abandonment varies from country to country. It can be as little as a few months or as long as two years.
According to MPEP, the US allows 6 months grace period after the deadline for a maintenance fee.
If inventor doesnot pay the maintenance fees then patent cease to have effect. As per MPEP 2590 Fee can be paid within 24 months and patent application can be revived subject to additional petition and requirements.
therefore even after not paying maintenance fee patent can be revived, however after expiry of 2 years from non-payment patent will be in public domain.
No its not possible to claim exactly same features, however improvements having novelty and non-obviousness can be patent.