I actually don't often use ReplayMod for sharing. Instead I use it for evidence against cheating. I often wish I could click into a player and see what they see. This would be fantastic for all uses cause you could share what other people see even if they don't have the mod (as long as they are rendered of course), AND for people like me because it would make catching killaura and a ton of other cheats so much easier.
Also, when I join a server, I can be on for hours at a time, and when I find something worth sharing it could be hours into a recording. That's a LOT of waiting for the mod to rehash all the packets, and a lot of wasted memory. So I'd like to propose a system similar to Nvidia's "Shadowplay" technology that simultaneously records your screen for a certain duration, and deletes captures older than that duration. That way a replay would never be longer than X minutes and you can skip right to the good stuff.
The first one is already possible. Have you actually tried "clicking into a player"? That should work, very similar to how it works for vanilla spectator mode. You can also press B and select a player there.
The second one isn't technically feasible for the same reason that starting a recording while already on the server isn't feasible.
If your replay becomes too long, you can use the Replay Editor to trim it down.