I've never noticed the second of two attacks always missing if the first one misses.
Silver Shields work like Mirrors and reflect the gaze from Medusae and Basilisks, so they are quite handy. Holy Symbols are only in PoR, I think, where they don't really seem to have any use. In theory they are supposed to help with Turning Undead.
Dexterity plays a major part in determining Initiative, which is partly why it is the most important stat. With 18 Dex for all your guys you are almost guaranteed to always have one of your characters starting the combat, which can be critical in the hardest battles, like the finale in Pools of Darkness.
Initiative is recalculated for each combatant when a new round starts, in my experience.
Note that in FRUA, unlike the GB games (except special areas like the Tilverton Sewers in CoAB), it is my experience that monsters get a +2 or +3 global modifier to all their dice rolls, including initiative. You will often see half the Golems, Ogres, Trolls, Giants and other slow (Dex 10) monsters move before your 18 Dex guys.. This has really soured my enthusiasm for FRUA.
-Is there an efficient method for forcing unique random encounters like "lairs"? Anything better than traipsing around the overworld like a decapitated fowl?
You mean the lairs on the overworld in PoR? No idea.
-What exactly determines whether or not the party is surprised in a random encounter? Are there any ways the player can reduce the frequency of said occurrence? I can't really tell whether or not having Search Mode toggled made a difference or not.
Seems to be totally random for the random encounters on the overworld (where Search have no effect) in the later games.
IIRC, In PoR areas have a "pool" of encounters, some of which will have you surprised, so surprise in these cases is "scripted". Once the pool is empty the area is cleared. Search mode will only determine how fast you trigger all the encounters.
In FRUA surprise is always predetermined by the module designer; there's no random chance. Probably the same for most Gold Box encounters.