You 're right, no cleric uses soothe because as it stands it doesn't do enough. Hence my suggestion that it reduce all healing aggro to 0 at rank 3. It would still be tough to take over something else, but it would at least be useful.
Perhaps some might seem a bit excessive at first glance, but I feel they would be beneficial overall. The gf needs to be able to maintain aggro and this fix does that very well. As for the dc fix, they have a number of useful class features so it would a difficult decision to take the newly improved soothe.
Knight's challenge is similar to what I'm suggesting yes, but I suggest the damage bonus apply for any of the gf's party members as well, with no drawback to the gf. That way it stays a useful core mechanic rather then a situational ability.
The mechanic is fine in theory if a little lackluster. Simply make any target marked receive some percent additional damage from attacks. This makes attacking the gf a good idea rather then his party members, it'd even work somewhat in pvp.