nice fish Zabber !
Spawn Sack - all your questions just confused me............which means I think you may be over thinking things and possibly confusing yourself. Your line is a FULL LINE intermediate sink line ? if so then yes you only need to add a leader, tie a DSL into one end and loop/loop it together then just tie your fly to the other end. salmon fishing you never need leaders over 6-8feet, what this means is you don't have to bother using tapered leaders because they will turn over being so short.
Now if your line is a DRY line then you will want to add a sink tip. loop/loop connect the sink tip to your dry line, add a butt section of heavy mono to the opposite end of the sink tip, tie a DSL loop into one end of your leader and loop/loop it to your butt section then just tie on your fly
If your using a full intermediate sink line for river fishing your probably working harder than you need to unless you have a stripping basket that is. I say this because your line laying in the water will sink making it tougher to get it up/out of the water to cast, This will cause you to make more false casts and fatiguing your shoulder. If you use a dry line with a sink tip most guys make 1 or 2 false casts and then "shoot" whats left, if you have a basket then 1 false cast is enough because there is no resistance on the stripped in line meaning it will go like gang busters
If you really want you can try adding a swivel but like you said 25lb test wont knot through a tiny swivel which means you'll need a size10 or larger which WILL add weight to where its tied in and could cause your fly to travel strangely. On lakes we only use swivels when fishing under indicators for 3 reasons, adds weight to get down faster, adds weight to keep your presentation straight up/down, adds weight to keep your presentation "in the zone" when fishing windy days or if you finger roll your fly in
don't cut off the loops on your fly lines, they make life easier. only line I would cut if off would be a CLEAR sink line