Alolan Raichu

If I have an Alolan Raichu with Speed Energy attached or a Starmie with Wash Energy attached, if I use their "Electro Rain" or "Multishot Star" attacks and discard those Special Energy cards would it get through Duraludon VMAX's "Skyscraper" Ability or not?

Because you don't know how many targets to select until you discard all the Energy, the process is to discard the Energy first, then choose which Pokemon take damage, and then apply damage. Since for those particular attacks the discard happens first and THEN damage is applied, as long as there are no other Special Energy cards attached to Alolan Raichu or Starmie the damage would not be blocked by Skyscraper.

When using Alolan Raichu's "Electro Rain" attack or Hoopa GX's "Devilish Hands GX" attack with a Telescopic Sight tool attached, would Telescopic Sight boost the damage to Pokemon-GX and Pokemon-V each and every time I choose that Pokemon?

Per the rule book you make all your choices first, calculate the damage, then apply modifiers (such as Telescopic Sight), and then apply damage to the chosen Pokemon. So you would not get to add +30 more than once if you choose the same Pokemon again and again; however if you choose multiple Pokemon-GX or Pokemon-V, you would get to add +30 to each of those.