As far as I know for all countries involved, the sniper rifle was a special form of the bolt action rifle. Now were Stripper clips used for feeding the rifle or were they reloaded manually bullet by bullet?
I don’t know the answer. However, in The White Sniper, Simo Hayha talks about carefully loading his rifle and making sure the bullets line up precisely.
Depended on the rifle. Some rifles like the Kar.98k had scopes that went over top of the chamber, meaning that bullets had to be loaded one at a time. Others like the M1 Garand (there was a sniper variant) had the scope off-centred so you could use a clip. Magazine fed rifles like the Gewehr 43 you could use as normal.