Coaches from East Croydon would be a ridiculous idea given there is no speedy arterial route south from there to Gatwick. It would be 20-30 minutes before the coaches were on the M23.
They don't really have much choice about using Redhill for this. Purley has narrow steps (although a reasonable bus turning circle). Coulsdon Town isn't particularly accessible to lots of coaches. East Grinstead works and has reasonable interchange facilities but the combined journey time is longer than it is via Redhill.
East Croydon is only really used overnight for coaches, when there are few reasonable alternatives.
Coulsdon Town has been used before for buses, with positive reports from all concerned. Purley is also a regular contender, and probably the best for off-peak times when it comes to a combination of 24/7 staffing, A23 access and bus facilities - though access can be a nightmare if it's anywhere nearing remotely busy on the roads.
East Grinstead usually works OK, but needs careful crowd management to avoid people rushing headlong across the station as trains depart.
The point about Redhill is really that it's as far south as possible in the circumstances, and therefore a theoretically shorter road journey. You'd have to run local bus services anyway, for customers wanting to travel from Redhill itself, and the line would remain open in this case for trains on the Tonbridge line. Problems are almost entirely avoidable, even with the less-than-perfect subway, and I believe lessons will actually have been learnt from the previous catastrophe.
The Sutton route was scheduled to be closed due to the resignalling project, but this has been postponed, so trains running hourly vic-Horsham-Gatwick in addition to the Redhill + buses option on Sunday
Very unusual in that those trains will be running non-stop from Victoria-Horsham. Understandable, though, if they can only put an hourly service together, which could be heavily loaded if those are the only trains advertised to Gatwick.
Unfortunately it looks like buses are still the only planned option for some of the intermediate stations from Dorking northwards!