Most increases are additive, most decreases are multiplicative.
Pretty sure graviton beam effect would boost reaper damage from 50% (HE does half to shield, not quarter) to 55%. As for stacking with those other things, I don't know. I know there are multiple methods of applying damage increases, like based on hull is different from straight damage boost, so it depends on what the effect in question uses under the hood.
In what scenario are you comparing salvaging? Post combat or campaign layer? I think salvaging and salvage gantry add together in campaign. There's a breakdown just before you salvage. Post combat is also probably additive. So with one salvage rig, you'd get 75% bonus salvage and 25% post combat salvage.
All range increases are additive unless they say they modify base range. % range increase work off the base range. For a 400 base range pd laser, pd mastery adds 200 range, advanced optics would add another 200 range, and capital grade itu would add 60% of 400, or 240 range, because neither pd mastery or advanced optics are base range increases, for a total of 1040 range pd laser. Gunnery implants would give another 60 range since it works off base range just like ITU. If you were to use high scatter amplifier instead of advanced optics; 400 base range reduced to 300 because of HSA, +200 from pd mastery + 60% (180) from ITU + 15% (45) from gunnery for a total of 725.
Compare to a light auto cannon being buffed by large slot ballistics range finder and capital ITU. The light auto cannon has 700 base range. It gets +200 base range from BRF. Then the ITU uses that new base range of 900 to increase the range by 540, to 1440 range instead of using the 700 range for 420 boost.
Safety overrides comes after everything else. For the AO pd laser, range would get reduced from 1100 to 612.5; for the HSA laser, 725 down to 518.75; and for the LAC, 1440 down to 697.5.