I will let you know a few things that I have learned over the last couple of months playing the game. I'v been playing for close to three months and I'm now at level 82. I'm far from an expert but....
To start, I would be selective on what heroes I dump time and resources into....so your already asking the right question. As a good guideline, I would look at each hero's profile on this website: http://soulhuntersdb.com
The site above gives different grades to every hero for each area of the game. With that said, there are only a handful of heroes that will excel in every part of the game. Moving forward, I would suggest focusing on leveling a strong top 5 (for Arena) while building up another 10 heroes (for Epic Arena which is like arena but its 3 battles...so you will need 15 heros total). The 15 heroes should allow you to do fine in Raids, Hall of Legends, and Crucible of Fire, and mining.
Make sure you are logging in every day and doing all of the daily quest to continue leveling. Additionally, if possible, mine as much as possible each day. For me, I mine for 10 hours in the early morning, 3 hours after that, and another 10 hours to end the day.
For the Crucible of Fire, if a battle is not going your way, you can exit the battle without any penalty. I wish I knew this when I was at your level...you just have to leave the battle before all of your heroes are dead. Furthermore, if you are going to spend more money....try to get to VIP 10. You can do the Crucible of Fire two times a day if you get to VIP 10....and you have the option to sweep the second time through. You won't get dragon scales for your second play through but the gold and items are worth trying to get to VIP 10 in my opinion. On a side note, wait until there is a double purchase event to purchase more diamonds. Don't buy the diamonds during a gold chest event or a item event...you can always get gold and items...but usually the double purchase events get you soul stones of great heroes.
On that note, I would be very selective with how I spend my diamonds...I wouldn't spend diamonds on items or soul stones. I spent a lot of my diamonds on chest but I kind of wish I didn't. Now, I only use my diamonds for buying stamina; I would buy more stamina everyday until it cost 200 diamonds to reset. I think you can do 2 purchases @ 50, and 4 @ 100...but I am not positive. Save the rest of your diamonds for special events.
For the guild, if you are in the second best guild you should be fine. It is beneficial to be in a top ranked and active guild so if for some reason your guild becomes inactive I wouldn't hesitate to join a different more active guild.
Now for your specific questions:
For your heroes to use your word...here are the keepers IMO...but again..I'm no expert.
Gizmo 3* (keeper) * Garrick 2* (keeper) I have both of these heroes but don't currently use them...with that said I know they are pretty good and probably better at higher levels then my current level
blood spear 2*
Prince of Persia 3*
Flora and Jasmine are the two main healers....Radulf does some healing. I used to be worried about healers but...while I love my 5* Jasmine her main use comes in the Crucible of Fire...I don't ever use her in raids, arena, or HOL....you can always hire a Jasmine for the Crucible of Fire.
For the shops, I feel like the most useful soul stones are found in the Crucible Supply Wagon..there you will find Zem, Taurus, Kong, LingLing.
For the legendary shop (i.e. hall of legends)...I only use them to buy Malrath stones...you will find Gizmo there too if you decide to keep leveling him.
A general tip for shops: Buy every item that you can buy for gold...even if you do not need the item you can use it later for enchanting items.
To help your guild, just do the raids each day (when you get to that level) and make sure to turn in your co-op points when you help others in the mystic ruins. Additionally, do not do your own mystic ruins...if your guild is active, you should have no problem getting three people to complete your mystic ruins for you...if others do your mystic ruins it will turn into more Guild Points for your guild.
Finally, my last tip is to not be in a rush. You got to level 40 in two days but I've been a disciplined player for almost 3 months and I am only at level 82. Right now, with a half of a day's worth of mining and some gold...I can level up 15 of my spare heroes to level 40 if I wanted. The real grind starts around level 80 but your progress will still start to slow now around 60-65. Be selective with how you use your resources...again check that website before you decide to level up a hero...you can always power level a hero later on with your saved up resources. In short, save save save.....gather items through shops and the crucible and save enchanting dust. I know it can be tempting to raise your hero's rating by enchanting their items but I would try to wait until ATLEAST purple 3 or 4 if you want to start enchanting items to full enchantment. Otherwise, you are going to use your dust, items, and gold to enchant items that you will only use for 2 or 3 days max before it will be time to fuse your hero again.