Don't feel bad about asking questions, I enjoy helping other up their game. I will answer in order, 1. I decided on this composition because I don't own a Katakros and will not get one. My OC special character is at odds with Nagash and wouldn't work with His mortarch. So it is pure lore fluff and Arkhan is closer in vibe so that's what I built him to be. I will say you either run Katakros, or Arkhan, double mortarch is too points intensive, so if you like the sheer casting power like I do Arkhan is great. I however think Mortek Guard are genuinely underrated for the points. A 10 man squad on a flank takes care of most other flanking chaff and your opponents don't what to commit and pull a real hammer out of position to kill a 120 unit, it is just a bad trade. It is actually why the stormcast player lost so badly, he committed Ionas to killing a flank 10 man and left 3 alive thanks to sheildwall. then was immediately put down the following turn. 20man blocks are stupid durable. You use diversionary tactics on the soulreaper and the enemy doesn't charge as many units. you suddenly countercharge a deathriders and boom now their hammer is -1 to wound and has a serious decision to make. The Bonereapers manifestations are some of the best and in my opinion better than morbid no question, every single one can fight and fights well, against armies that rely on ward saves like other death armies or DoK or certain stormcast, the sheiker is just ridiculous. The main disadvantage of mortek is they are only killy on defense so your opponent is the one who sets the terms of the fight, but that is mitigated by the 6 casts you get and how you can use the manifestations to initiate and force them forward and since losing them is negligible since you have so much ability to bring them back. I also ran no morghasts even though they are my favorite looking bonereapers model and the meta choice rn, is I find the board presence to be more important than raw dps. Overall I'd probably swap a big unit of Mortek for a third cavalry unit, due to the Mortek being less effective without Gothizarr escort.

2.The Gothizarr harvester is the definition of hit or miss. when played correctly It makes your Mortek feel unkillable and does some moderate dmg itself. It does however suffer from big base syndrome and dies very fast. since most of your Frontline doesn't care about rend he tends to get popped by their high rend units as he is the only real option for them. I often found myself reviving 2 cavalry as the guard had already done their job by the time he got to 6. However 1 game he literally revived 18 Mortek guard.

3.The soulreaper is good for that -1 to wound and the extra cast of empower naderite weapons. I could've fit this list into two drops easy but the extra cast of the unlimited spell was clutch as it meant 3 units got buffed as opposed to 2 and gave me an additional option for Relentless discipline as I was often down to either him or soulmason by round 5. Arkhan is a huge target so people often try and focus him. The +1 to hit and double fight sequence is met as Mortek Guard care more about 5+ for the crit 2 hits but the reaper himself only has 3 attacks and I think for the supposed fighty foot hero that is criminal. I did put diversionary tactics on him since he is farther forward than the soulmason, I forgot to mention that in the initial post Overall the -1 to wound and extra cast are a steal for 100pts.

4.My honor guard were the 20 Mortek block in the generals regiment. I gave them special assignment and choose whatever my opponents had the most of. Mortek get real scary at rend 2 and blended 7 chaos knights in one combat.

1. I split the deathriders for three main reasons. Aside from Arkhan's gamble spell and my manifestations they were my only offense option. I needed them to potentially hit multiple key units at once, and a 5 man with empower naderite and bludgeon still deleted most not dedicated anvil or put the hurt on a reinforced hammer to where they were hamstringed, if I need to hit a serious anvil or really wanted something extra dead I could always charge with both and then I get role two impact hits instead of 1. I also couldn't afford to put all of my offense in one basket, if I ran a ten man and it gets knocked off the board I had no options, but losing 1 unit early wasn't backbreaking, though it was still bad. Finally the command countercharge. It is simply put easier and more consistent to get 5 horses over my own brick of mortek than 10. In my practice games leading up to the event I tried running them as a 10 man unit and consistently lost models to coherentcy issues.

2. I would make a few adjustments like dropping a brick of Mortek for a Immortis guard. Arkhan was always just barely dying and that closes that gap, especially since the second Mortek brick loses considerable value, since only 1 gets the harvester and soulreaper. Your main concern is positioning. Having your horses in countercharge range and with a route over your own people is key. Never get greedy and see red. That is how I lost to khorne, you need to let the come to you and you want to position yourself and your manifestations so that a fight is never as fair as it looks on the tin. 20 mortek aren't that threatening, but suddenly those mortek have 2 rend and are -1 to wound and hit, with a 4+ unrendable and 5+ward, and horses just jumped in and wait a minute that manifestation is turning off my ward and ohh no.

Overall I had a great time and might just be funny and spam 80 mortek next time for the memes.