Solo-related
a) Wondering how you ended up with an IFP of 7? The best you can do is 2 with the infantry units present in this scenario (they all have a IFP of 1 and combining 2 or 3 units would result in a combined IFP of 2). Of course, adding a couple of MGs and a leader could bring the total FP to 7 easily
b) Being two hexes away from an objective building does not mean you have to skip a Priority-2 order. You have to be more than 2 hexes away from a PU to do so, and you would proceed to a Secondary order only if there is no Priority Order-3, or you have to skip this one too.
c) Did you go through the Combined Fire Action Flowchart first, if you were firing a stack (the only way to reach a total FP of 7)?
d) Sniper cannot move so PU could not be spotted by a Moved marker but, for the sake of your question, I will assume it was spotted for some other reason than Moved or Fired (as you mention possible Opp fire)
e) Considering that a lone sniper in a LC building hex has a TM of 6, firing at it with 7 FP is not such a bad idea but yes, depending on the tactical situaiton (other units able to opp fire beside the sniper for example), sending one unit to expend the sniper's opp fire , then, in a subsequent impulse, sending another unit to eliminate the sniper in Melee might be a better solution and it is indeed a situation where you have to use your brain soldier!
f) You are right, I think the Solo system does not "split" his own stacks; it,s the nature of the beast
Generic
1) The default rules is that building only have a lower (ground floor- Level 0) level, unless they are of the Heavy Construction type AND bigger than two hexes. I'm assuming this SSR was put in that scenario so people stop asking if there was a usable steeple in the church building (which would, at least, be a upper level hex)
2) As indicated under 4.1, a unit marked ops complete can fire at a unit it just spotted, and other units in its hex can join (you actually remove the ops complete marker in this situation, replacing it with a Fired marker). If it fails, it stay marked ops complete and thus cannot do anything else beside opp firing later in the turn (and only if the unit is a MMC). Other units in the hex of the spotter could then be activated in this, or a later, impulse. Note that you can spot from one hex and, if successfull, fire at the just spotted hex with a stack from a different hex than the spotter's hex, who would then be marked ops complete.
3) The hex with the Melee marker is automatically spotted (see 10.0) but you can't fire on a Melee hex nor fire from a melee hex
4) The SW is dropped in the hex and will be recoverable by the enemy unit next turn, provided there is no more Melee going on in the hex