Yes and no, thoughtful.
3000 metres height gain ? No problem. 5 hours ? Fine. But a landout 50km away or more would require a bronze badge and cross country endorsement as a pre-requisite to setting off outside gliding range of the home airfield. And the bronze badge itself requires a certain minimum of solo experience so, no, all three silver legs cannot be legally acquired on first solo...
Yes entirely actually as far as I can see!
There is no LEGAL requirement for a bronze badge and cross country endorsement before flying cross country, they are simply BGA rules.
AFAIK, there is no FIA requirement to hold these first. Unlike the Silver, they are not issued by the FIA but are a UK oddity due to the lack of a formal licence (at the moment).
So, if you somehow make such a flight and an OO confirms it I can't see there is a LEGAL problem.
All this is rather theoretical and it is hard to imagine it actually happening!
I think I am still an OO (or does it lapse with lack of use?). Perhaps the OP would like me to come and observe!