Depends on who you have. The best thing you can do to progress is to farm campaign and get your starter champ to six stars and level 60. Then repeat with an AoE Decrease Defense like the campaign farmable Warmaiden.

I would say no - don't waste resources on any of those.

Also - I would highly recommend you stop using your shards on normal days. Every other week there is a 2x weekend where you double your chances of getting epic/legendary champs from a specific shard type. For example, this coming weekend will be 2x on Sacred shards. Wait for those 2x weekends to use those shard types.

Ah, thanks, did not know that!  I was completing the daily or weekly quest to summon champions- some are from today, some from another day and I was just looking at my Rare  higher stars.

I am assuming no deals on Mystery Shards so I can use them any time?  I'll stick to summoning with those to complete the quests/challenges.

Correct - though it's still interesting to stockpile mystery shards as much as you can, because at the same time as the double summoning rate events, there are also Champion Chase or Summon Rush events, that give rewards if you reach a certain point total from summons. And if you've been slowly building a stockpile of mystery shards so you have a couple thousand of them, it means you can reach those rewards without having to use other shards. 

So basically, use mystery shards for daily quests or if you are running short on champions to use as food, but don't pull everything you have trying to get better champs (there is a very small chance of getting rares from a mystery shard so once every fifty or so shards, you'll get lucky).  

All of them will not be viable even in early midgame- Do not invest too much into them

OK, thanks - I was using them to complete quests and sacrifice to level up (one star commons).  I will just summon enough to complete quests/challenges for now.  That's only 3 a day and I get a bunch from arenas and dungeons - I have about 25 shards now and that's after total summoning since I started the game, of about 60.  I think I have a cap on how many shards I have?  I'll look once I'm done beating this Fire Knight.

pull junk rares. use them to rank up good rares.

Now to figure out who's junk and who isn't.....

https://youtu.be/19vqyqsF8CA

Try  this video,  these  are  the  rares  that  Ash  would  take  to  60.  You  can  also  find  other  guides  detailing  all  of  the  good  rares.