A character without Scout can join a unit with Scout, and still get the Scout move. It doesn't work with Infiltrate, but with Scout and Outflank you're good to go.
Well, firstly I believe you can only summon the Bloodthirster of Unfettered Rage, not the others. Second, to Summon it you're sacrificing your Herald, so you won't be Summoning all 3, and once you Summon one you aren't using any more powers from the Herald. Thirdly, you're banking on actually getting that spell, and even with ML 3 that's only a 50% chance, and your opponent can still Deny it or you could fail to roll it. As you said you want to play competitively, that means you can't really be basing your plan on a 50% chance. And finally, if you Summon the Bloodthirster it arrives by Deep Strike near your Herald, so it's arriving in Swooping mode (since it's a Flying Monstrous Creature), and unless you've moved your Horrors and Herald up close (which is a bad idea), it will take a turn or two to even get into combat, so it's not something to do late game as the Thirster will either not get into combat or only kill one unit.
So if you're playing for fun, try and Summon the big guy. If not, don't count on it because there are too many ways it can go wrong. A Keeper of Secrets or Lord of Change would honestly be better Greater Daemons to summon, as a Keeper will begin on the ground and is very fast, and a Lord of Change should be flying anyways while it casts spells. Both can also cast spells, so you aren't almost giving up the Psychic Phase once the Herald goes poof.