A few answers!
I'm not sure I agree a rec is an odd place for teenagers to hang out. It's light till 10pm-ish at the moment. If they haven't got much money and are active outdoorsy types rather than slump in front of the telly/laptop types, which these two were if you consider the urbex activities, then I think it might be a relatively normal place to spend time, talk, run into friends, maybe drink or smoke. There isn't much for teens to do in a place like that. Even the pub is going to be full of your parents' friends.
Check this out, which seems like exactly the sort of nonsense they might have got up to in happier times:
TikTok: Warning over teenagers getting stuck in play swings
Marc at least was reported to have cycled there.
If Lewis stayed in the local area overnight, he would most likely have taken a 29 bus to Crowborough the next day for work - the same bus he left on shortly afterwards in the opposite direction. If he used the bus regularly, he probably had a smartcard.
Why he went to Eastbourne and how LE knew where to find him is certainly very curious. It would be interesting to know what happened/was said in the hour or so he spent at work, and what state he was in if he'd spent the night away from home/out in the open. If his defence team have applied for psychiatric evaluations, one can only imagine his behaviour there was erratic, so perhaps a colleague tipped them off.
Brighton & Hove buses radiate out from Brighton. If Marc took a 29 all the way from Crowborough to Brighton, it would have taken him about 1hr 20 minutes, and if he then took a 12 out of Brighton to Eastbourne, the same again. (There are other ways to do that journey, but just as a thought experiment.) If police had a tip off from a colleague, they could literally have followed him, as the slowness of the bus would have given them ample catch-up time.
JMO