No idea about the diamond chest spawns, all mine get attention at least once a day. Seems to be same frequency of chests and diamonds across all accounts.
Have two accounts running together pretty much as you described. Started on the same day. The partner to my newly HQ19 single player, is a newly HQ18 very peaceful turtle. VP 253, but I only have that account at the matching radar of 16. Which seems to be a problem as my radar says "You must destroy 36 enemy bases to get new invasions". That account is out of NPCs, other than 2 Hammerman HQs, and the last one of those (had three) didn't count.
VP was easier to get on the turtle, my HQ19 single player is still struggling to maintain at least 170 VP every 24hrs (for two intel per boat).
The turtle has been attacked all of seven times in the last year, raided zero. I do use a survey app that pays me in store credit. So I have bought and used about 5k extra diamonds to get that account's defense midrange, back around HQ15?, during a crab event. Mostly speed ups then, because I figured it would get raided, and wanted intel/diamonds. Didn't happen.
Both accounts run attack statue lineups, and max troops, etc. The turtle struggles to keep crafts maxed, so usually is far behind on defense. Especially now, with the NPC radar thing, even with access to IG and Warfactory. I guess turtles must keep leveling the radar.
Not sure why it doesn't get raided, because I've done challenge attacks on it, and so have friends, within VP/HQ/level range, with no problem taking it down. Maybe everyone in that range is waiting only for a real whale, to make it worthwhile? I tend to spend resources down asap, because I plan out the upgrades according to average intake. It was getting scouted 5-6 times a day, but now it's only 1-2 every few days. Not complaining, but it's surprising.
(*PSA - The above described 'turtle' is not the best way to do that, it's just what is working atm, likely only because I have many accounts to keep me occupied. Attacking no PC bases with that setup is not resource efficient at all. Notice the entire HQ level deficiency. Having to time resource gathering to set times is not the turtle way. That this account can keep a surplus of anything overnight is strange. That account should be getting raided clean every day, and isn't. I'm not responsible for your agony if you try something similar.)