Timer (or other) interrupt happens while servicing an interrupt - how to accurately handle the second one?
Q1:


  ... exit the input ISR and interrupts are (automatically?) re-enabled


Yes, the RETI (return from interrupt) instruction enables interrupts.  See third paragraph of section 7.7 in Atmel's document #8271 (a 650-page spec sheet for the Atmega 48, 88, 168, 328 series), which says:


  When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is executed.


Q2:


  Assuming the timer is uncomplicated (it wouldn't overflow twice during my input ISR) how do I accurately handle the fact that it overflowed during my input ISR and I "missed" it?


If the timer is running a clock, set the timer control bits to automatically reload the count register upon each overflow (or underflow).  That will leave a full count period for processing each timer interrupt.

That is, you can pre-load a next-timer-count (I think it can be different from the current count if necessary), so the timer can start its next interval automatically.  If the timer is used for some purpose more complicated than that, you may need a nested interrupt.

Q3:


  Would this differ if instead of the timer I simply had a second input2 that similarly might fire during my input1 processing (but would present a useful input long enough to process after input1, presuming I knew it triggered)?


If indeed input2 sticks around long enough to process after input1 processing is done, no problem.  Its interrupt will occur after the first interrupted code sequence executes one more instruction after the RETI occurs.  (See paragraph 6 of section 7.7 in doc8271.)

If input2 is brief, several different things can happen.   (a) If it is shorter than a clock cycle, in general it won't trigger an interrupt.  (b) If it is a level-interrupt-triggering event on INT0 or INT1 that ends while another interrupt is being processed, it won't trigger an interrupt.  (See discussion on page 72, section 13.2.3, of doc8271 of INTF0 and INTF1 being clear for level interrupts.) (c) If it is an interrupt-flag-setting event that sets a flag and ends while another interrupt is being processed, it will trigger an interrupt after the global interrupt flag is set, unless current processing clears its flag.

I think the above accurately states what happens on Atmega devices, but I suggest you also see doc8271 – and run tests – to see for yourself.  Here is part of what paragraphs 4 and 5 of section 7.7 in doc8271 say regarding (b) and (c):


  There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt Flag.  ... Interrupt Flags can also be cleared by
  writing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the
  corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the Global Interrupt Enable bit is set, and will then be executed by order of priority.
  
  The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the
  interrupt will not be triggered.