Hey kilo,
I'll try to answer your questions to the best of my knowledge
1) If you leave it blank the FMS assumes a no-wind situation, (regardless if the airplane has the geometric descent option or not, the TOD and path will always be computed for a constant Mach/IAS idle descent unless there are altitude restrictions on the path that would make a level-off necessary, in that case as you know the ones without geo-descent would actually fly level until reaching a new TOD to perform an idle descent to the next altitude). Similarly it will not apply any correction for local QNH or ISA deviation, during descent the path will also not be recalculated using actual values.
2) Same here, if you don't enter QNH or TL the FMS will enter the standard TA of the destination airport from the airport and assume standard QNH, in your example for question two the FMS will be somewhat surprised that you are low on path once you set 980 at TL and add thrust to get you back onto its computed path with 1000fpm.
My $.02: Entering all info is great to get a good estimate of a continuous descent TOD, but once vectors or ATC restrictions come into play you can basically forget it unless you can anticipate them and also enter them early enough into the box... My personal technique is to put the FAP into the 'descent target box' (or whatever its called @ 4R on the Descent page overwriting any other restriction it might display) if its not already there together with its crossing altitude to see the
VB and VS needed for an immediate direct and plan with that in parallel to the VNAV path...