I`m not 100% sure about this time round, but in the past, ATW have put replacement buses on where possible on a Sunday. They dont do it for other days of the week where there is industrial action, because basically, they dont have to. Services cancelled on Sundays due to lack of drivers is not classed as industrial action.
I`m pretty sure the RMT drivers will not cross the picket line, as when there was a strike recently by the RMT, the Aslef drivers did not cross their picket line.
As for other grades not crossing the picket line, it wouldnt and shouldnt happen. Most of the guards and other staff will turn up to work, sit around for a few hours then go home. The whole point of a strike is to hit the company where it hurts. If guards and other staff did not cross the picket line, it is doing the company a favour as they dont have to pay them, and with the guards also being in dispute with ATW, there is no love lost at the moment.
Staff not involved in the strike have been urged to take an annual leave day on the strike date ( seriously, who thinks this stuff up and puts their name to it..... its so laughable ) and staff who have carried over annual leave from last year are being FORCED to take annual leave on the strike date. Im not sure if and how this is even legal.