The OP will not have time to try the journey out a few times in the holidays, as they will have been asked for a response re the higher preference school (presumably SHSG) well before the end of term. Perhaps try it tomorrow, if the primary school is closed through the NEU strike action?
Two of our DC attended SHSG and for us, it was an easy decision to accept when DD was offered a mid-year place at the end of year 7, as it had been her first preference and we were familiar with the school. But we are only a few stops down the C2C line, so no issues with taking up any after school opportunities (or even, later on, the odd detention - which we were warned, as the girls went into year 10, would not be waived just because the miscreant faced a difficult journey home afterwards).
As for 'trying out' SHSG first, the reality of pressure on school places in the Chelmsford area is that if that doesn't work put, the local solution will probably, if anywhere is available, be Hylands, certainly not Moulsham.
We loved SHSG, but I'm fairly certain i wouldn't even have considered it as an option from Chelmsford (I took DS2 to the open morning at KEGS in his time, just to see what all the fuss was about - he liked it, but probably for its proximity to Lek's in the market for lunch, and rapidly went off the idea once he'd processed the reality of the journey there and back, daily, for 39 weeks of the year, for 5 years).
Perhaps a better way round would be, Moulsham High to GCSE, then consider SHSG (or CCHS? Or KEGS? Or WHSG?) for sixth form?