Diviner actually has a lot more utility than it seems. It's a level 2/6 tuner (through its effect) that allows you to make an early baronne before committing to gallant granite or access halq or the rest of the synchro pool in desperate situations, can send N'tss to guarantee baiting interruptions or outing floodgates, and also provides the option to search by sending arc light as you mentioned. Manju is a fine replacement, but you simply miss on all the versatility diviner provides.

Halq being banned actually doesn't change this list much. Instead of the usual routing, you'd use researcher+level 4 megalith for coral dragon, block+cross-sheep for union carrier, then coral dragon+weeping idol for savage. The only difference here is that you can't afford to have block dragon as part of the end board of baronne+savage (equipped with buster) which is kinda whatever honestly.

So I'm going to throw out a lot of theory on these points that you may or may not agree with which is totally fine. Gigantes as an extender pre-gallant requires setup aka it functions as an unnecessary additional soft brick without ritual setup. The only reason to really include it would be to replace weeping idol for it making an additional level 10 synchro instead of borreload savage; changying being the only other decent generic level 10 but isn't great in a megalith focused build. x2 candoll is very much a similar idea to x2 o-lion in halqdon builds. However, it isn't as severe of a detriment and hence the second candoll brick isn't necessary. Combo pieces such as level 8 megaliths, diviner, extra foolish burial, talismandra (off of chalislime), inception, etc and the ways to search them all bypass opening candoll and still net you full combo. As such, a second candoll is actually more of a detriment (additional brick) than it helps as you play around opening candoll in hand regularly. Finally, in a combo-focused megalith build w/block dragon, cards such as emergence provides some notable anti-synergies. It's an additional brick in a build with some soft garnets, it's function as a grind tool competes with block dragon for gy resources, and doesn't provide as much followup in the opp turn as revived phul doesn't lvl modulate or get back a ritual. In fact, searching aratron and phul as part of the regular combo does what emergence does but better in these megalith combo builds specifically. They are combo pieces that don't brick, aratron in the opp turn enables phul's lvl modulation and ritual retrieval (which then enables impcantation followup if not done previously) and activates in hand which dodges baits through back row interruption. Feel free to experiment with your suggestions however and see what you enjoy most; that's the goal after all.