1) afaik no, but immigration lines are usually short enough, not to worry about it
2) sq terminal 3, first class check in and priority immigration line is all the way to the left side, opposite side of jewel/skytrain. As long as you have your boarding pass& lounge invitation, you can skip the checkin, just go to the priority immigration line, which let's you out very close to the T3 lounge
Note that T3 The Private Room is scheduled to be closed for renovations sometime 2020
https://www.flyertalk.com/forum/31838510-post4.html
Map, click on T3, level 2. Sia first class checking is all the way on the left side. Priority immigration isn't shown, but it's just north of the first class checkin
https://www.changiairport.com/en/map...103.986424/-67