Apothecary will help you basically everywhere, also arena and CB, replacing Diabolist indeed. Taking him to 60 is a solid move. 

Ursine Ironhide could still be useful in dungeons, he might be worth 60 really, though I don't have him built myself. 

Occult Brawler is absolutely worth 60, though he's pretty specialized and you'll mostly just use him on Clan Boss. Potentially Dragon at some point, once you can afford to bring a specialized champ like OB who contributes little or nothing on waves, only on the dragon itself. 

And forget about Doom Tower - you shouldn't take it into account for any of your champion building decisions at this point. Dungeon progress should be your priority, with arena and CB progress as secondary priorities. DT is the least of your concerns - any rewards you can get to, great, otherwise so be it. 

As for the fusion - honestly, it will be VERY tough with your level of dungeon progress. You'll need to do crazy amounts of grinding to get enough points in the dungeon events, on top of the crazy amounts of grinding to acquire, level up and ascend the champions, plus of course the shards needed to get enough points in the champion chase and summon rush. Both in terms of resources spent and in terms of sheer time spent, it would be super intense and honestly I'm not sure you should go for it at all. At least not for the legendary - the one advantage of regular fusions over fragment ones, is that you may be able to fuse one or two of the epics relatively easily, which could already be worth it if the epics in question are good. 

Keeping in mind the current situation in arena, I would say level your champs only if you want to. i would stick to getting better artifacts and speed tune your team, finish all masteries and get some accessories from spider and get scrolls from  minotaur. and use rest of the energy on missions and challenges. this way you will kee power down and keep speed tuning your champs and better chance of moving to silver 1, although i would still say speed tuning is a key task if you want to reach silver 1

Yeah OB will be used in the future probably but I feel iam a little bit to far off from specializing that much as you mentioned.

With regards to the fusion. I feel like this is going to be a challenge yes but it might be worth trying. Its not like it wont help my other champions get stronger either. At this moment I have around 2700 energy in my storage saved and 800 gems. That should help me a lot with farming and Iam saving my daily energy pot as well so that should be another extra like 700 energy + more gems (I buy the daily gems pack). Next to that I now have 40 ancient shards, 1 void and 1 sacred. Hopefully this helps some but this it what worries the most. That you will need like 10+ sacred shards to get the fusion rares and stuff :)

Time is not really an issue. I have time to play so dont mind the grind. I might want to stockpile some on dungeon potions maybe. Dont have a ton stacked there atm for the ascending. Not sure if that is going to be possible after the event or that it is really time limited

Yeah that is basically what iam doing now but that feels a little tedious (for lack of a better word). Main goal on that regard is getting better gear on Kael (4star epic lifesteal gear now). Have been farming dragons lair a lot with the event (got first in my group aswell) but hardly any luck on good gear. Also same for spider and no luck with a banner on him. Did like 100 runs or more but yeah.

In that case, yeah, you'll definitely need to stock up on dungeon potions in advance so at least you don't have to worry about that anymore during the fusion. If you can get any unrelated fusions ready to go, like Broadmaw or Justiciar/Relickeeper, that could help a lot with the champion chase and indeed you'll need to keep all your current shards - stockpile mystery shards as much as possible, as well. And stockpile silver - you'll need tens of millions of it, obviously you'll get a whole bunch while grinding XP during the event, but it helps to go in with a lot of cash already. 

For the rare in the summon rush, you usually need somewhere between 1000 and 1500 points, which means 2-3 sacred shards, or you can replace 1 sacred by 4 and a bit voids or 50 ancients or 500 mysteries, or some combination thereof. But ancients/voids are better to use in the champion chase than the summon rush, considering that the champion chase will reward you based on the champion you pull (so getting an epic from an ancient shard is immediately 250 points), while the summon rush is based on the shard used (even if you'd get a legendary from your ancient shard, it still only gives you the points for an ancient shard). If you can get any champions from fusions or login rewards during the champion chase, it can cut down significantly on the number of shards you need. 

And then during the event, I guess I would say, know when to cut your losses if it's too hard. Go for it from the start, but if you find at some point that you really aren't getting there and have already exhausted all your gems and resources with a bunch of events still to go, then maybe change your goal and see which epics you can still get. It would be very frustrating to go all-in for the legendary but then end up falling just short.