Hello
RAM Voltages
1)If a MB is listed as 1.8v, and the RAM in question is 2.1v, will the RAM work on the system, considering the RAM meets all other qualifications?
YES the ram sticks will work at 1.8Volt / its desirable for ram sticks that are advertised at 2.1 volt because the high ram voltage is needed for overclocking
2)Same question, but with the voltages reversed. (the motherboard is defalted to supply 2.1 volt vdimm to the ram but you are going to use ram that is stated to be 1.8volt)
thats not so good IMHO and I would avoid that senario
3)What dangers, both physical (frying of components, etc) and theoretical (slower system, memory errors/crash) are likely to result in these two scenarios?
The 2.1 volt "rated" ram will run just fine at the stock voltage of 1.8volt / but if you want more perormance and overclocking it will need to be bumped to 2.1volt )
MB Power Connectors
1)MB uses a 20pin power connection. PSU has a 24pin connector (not a 20+4 that are hooked together but are easily separated). Can the 24pin connector be plugged in with 4 pins overhanging, and still power the board?
Yes / a 24pin PSU connector can be used without any negative effects / simply overhang the four pins off the 20 pin connector / no need to worry about orientation / the snap lock on the 24 pin power conenctor will still align with the 20-pin connector 
2)Same question, but the pin counts are reversed (MB-24pin, PSU-20pin...can I leave 4 unused?)
YES you can leave 4 pins empty / the purpose of the four pins depends on your video card selection and IF you are using SLI (dual video cards) the extra four pin power is the pci express video card power feed to the second PCI express power slot & can be used for the bigger voltage drawing video cards / if you are trying to run a top notch video card (one that needs a 6-pin pci express video card plug) then chances are the 20 pin PSU will not "pull the wagon"
3)If the answers to these are yes in either case, how do I determine which 4 to leave open/overhang?
the power plug will only insert in one orientation and still get a lock by the plastic snap lock
4)What dangers, in the same vein as mentioned above, are likely to result from these two scenarios?
the more desireable case would be the powering of a 20 pin board with a 24 pin PSU >>>> there is absolutely nothing lacking in capability here >>>> most likely the PSU is of better grade than the system needs / however
if you power a motherboard needing a 24 pin power connector with only a 20 pin AND you are using a high class video card; chances are your system will be power starved; this is not good 
SATA Cables
1)Do SATA1.5Gb/s and SATA3.0Gb/s hard drives use the same cables to connect to the MB? I have read conflicting answers to this.
2)If the answer is yes, why are there two different types of cables available for sale (SATA and SATAII)?
I will stick my neck out and say YES you can use the same cable for both, there are some "camps" of thought that cables sold as labeled Sata-2 have better shielding, I have yet to see proof of this >>>>> put me in the skeptic camp
enjoy and dont hesitate to ask more questions
best regards
joe