Hey there Yvain! I've been thinking about your list, and here is my BS and two-cents, in long rambly form:
Guard for me have always been about redundancy. While that means more than simply "taking two of every unit", I find that doing just that is a great stupid way to beat your opponent over the head. For the most important units, I have a rule of thumb --
if it's worth taking, it's worth taking two. By my reckoning, that will change your: Infantry Platoon, Veteran Squad, Fast Attack choices, Heavy Support choices, and your Scion HQ.
Another rule of thumb for me is to never give a single unit more than two firing niches/tasks in a game. So for a unit like your Infantry squads, I see them doing three or four things: 1) Sitting on Home Objectives, 2) Bubblewrapping Artillery, 3) Shooting Light Vehicles with Autocannons, 4) Shooting MCs or Characters with Sniper Rifles. I would limit some of their wargear, and thereby simplify their job.
Double- or triple-up on either the Manticore or the Basilisk for redundancy's sake. You can't expect any one tank to survive longer than turn 3 or 4 against an opponent who really wants it to die. Make his job harder and take two. Make it a hemorrhoid for him and take three. And, in my humble opinion, the Wyvern is trash. The current meta favors Flyers, Heavy Tanks, and Monstrous Creatures, all of which fall outside of the firing niche of the Wyvern. Beautiful model, though, IMO.
As far as your Fast Attack goes, its the same game as your Heavy Support. Take two of the same thing, not one of each. Just as above, any one vehicle can die easily -- having two of the same makes your opponent commit more firepower, freeing up scoring units to run around and play. I recommend two Vendettas, because this:
Linky(Also, if you double-up your Infantry Platoons, you'll have two Vendettas to fly your two Flamer-PCS around in)
And if your Valkyrie Goes, then so go your Veterans. You'd need two squads of melta-vets anyway to reliably ensure completion of Tank Hunting if they're moving around the board in an AV 12/12/10 vehicle, even if it is a flyer.
In honor of your lost vets, I would recommend giving the Scion Command squad melta instead of volley guns. A Scion Command Squad can order itself
to twin-link it's own 4 meltaguns. That's four meltaguns hitting on 3s and rerolling misses. Hard to beat that kind of AT. Make it redundant by dropping in two more squads of Scions with Meltas right along with them.
Words words words, blah blah blah. What else did I change? Cut your CCS for points, added a third servo-skull because they're awesome and you'll wish your had more. My advised revision looks like this. I think it comes out slightly under 1750, so enjoy the extra points!
--PRIMARY DETACHMENT—
HQTank Command Squadron
-Leman Russ Vanquisher w/ Lascannon (Warlord)
-Leman Russ Eradicator
Primaris Psyker
Inquisitor w/ 3x Servo Skulls
TROOPSInfantry Platoon (180)
-PCS w/ 2x Flamer
-2x Infantry Squad w/ 2x Autocannon
Infantry Platoon (180)
-PCS w/ 2x Flamer
-2x Infantry Squad w/ 2x Autocannon
FAST ATTACKVendetta Gunship
Vendetta Gunship
HEAVY SUPPORTBasilisk
Basilisk
Basilisk
--ALLIED DETACHMENT—
HQScion Command Squad w/ 4x Meltagun
TROOPSScion Squad w/ 2x Meltagun
Scion Squad w/ 2x Meltagun
I would really love to hear how it plays!
-Gunny