k i have not played this game though this is by far one of my most favorite games ever.
1. no answer sorry not enough detail.
2. i do beleive the max lvl is 10-15
Often i only save upgrades though for gear i really like in terms of ability and such as this game has a split focus on attack. yes a higher attack or defense is nice but sometimes a lesser gear can get an ability that surpasses higher gear due to functionality these are the gears that i save to upgrade along side my main set of max utility. example cyrille bows not fantastic you say but if you are in the training ground and you pick up a burning bow. due to its special charge versus its regular attack that is middling slow and strongish but charge is a powerful attack that even near end game eats alot of ememy hp. so only upgrade equipment that you plan to keep due to its abilities AND power.
upgrading serves a purpose of making a weapon's/ armor's base stats more powerful as well this is why you only upgrade item you plan to keep for an extended period of time. recommendation make sure to utilize the training ground yeah it gets tough in there but you can often get armor and weapons with specials not normally found and above the current quality lvl in terms of offense and defense. also to make upgrading stats easier keep a couple of the low items with the "mythril for kills" ability on it later every little bit counts even if it takes a little longer to kill enemies thus upgrading these low grade items makes them easier to use against higher lvl enemies cause it puts them closer in power to what your fighting though will never ever be able to go on par with your best gear.
if you must save an item for upgrading last is the rings and braclets but dont ignore them as for toma defence against magic is often found on these items and upgrades as you rank up the items lvl. cyrille should have a high defense item for magic and the others should boost her magic attack. Toma is not a mage he uses stun magic and push back for breather room. as most HIS MAGIC is reserved for charge attack
3. this is less obvious at the beginning when money is real tight sell pretty much everything you don't need and port often so you can continuously PICK UP EVERYTHING. towards the middle you start to notice they are dropping better quality gear now huh well sell anything that is not enchanted (extra bonus stats in the slots) appraise everything (yes you are going to lose money this way but you always stand a better chance of regaining it back with this method as a appraised gear will always sell for more) now head over and see what your gear is in the break menu anything that gives 800 mythril or above is a definite break if you are not planning on wearing (later you can if your are felling like it raise the bar or drop it as higher enchants become more common). this is due to they drop all this great gear a lot i mean it a lot and you are preventing character stat growth if you do not take advantage of this. now go sell everything else. notice though you sold all the bonus devoid gear ,appraised 20 or so items broke a few of them and sold the rest like you did the bonus devoid gear you now have more money than you started with.
use the training ground you get piles of gear and money from this gear nuff said
4. small tip the cannon upgrade useless as most this game happens out of range of cannon or underground in building ect. range is not determined by how high your cannon upgrade is, it is story line determined. put your cores elsewhere (like geo fortress shields, geo fortress hp regen, and did i mention training ground earlier yeah put them here far more useful as cyrille and toma eventually become genocidal walking bombs of destruction