If Soviet MG “C” used Ranged Fire at Finnish MG “A” and received a Suppressed result or better, would “A” be prevented from giving Defensive Ranged Fire to the Pajari stack against the Soviet infantry stack attacking over the bridge?
Yes; see the last sentence of 10.2.
Or would this have no effect on the combat over the bridge and simply prevent Finnish MG “A” from using Ranged Fire offensive support during the Finnish turn,?
Note that Suppression markers are removed at the end of the combat phase in which they are placed (10.4.1).
And while I am just listing it all out here, would Soviet MG “D” have LOS to Finnish MG “B” or do those small islands and bit of land on the hex side obstruct this?
They do have LOS; see 5.2.4, or the second sentence of 5.0.