Considering a day trip to Brighton on Friday I have a senior rail card, am I correct in thinking a paper one day zones I-9 travel card rather than using an Oyster card with a discount loaded and a boundary zone 6 day return is the best option?
How is the best option defined, and how does one go about determining it?
Rickmansworth to London Zones 1-6 (AAA: Zones 7-9) is £8.60 with Senior Railcard discount.
Boundary Zone 6 to Brighton is £13.15 with discount.
You can use any train, including the lightly loaded services which the company brands "Express" (but, despite the name, are 1 minute slower than their "Southern" branded trains from Brighton to Victoria), which at off-peak times you can generally get a carriage to yourself (but at peak times are uncomfortably wedged).
If you prefer to use Oyster, it's £3.55 each way
(if you can travel off peak both ways, and ignoring the slow option of changing at Northwick Park, Kenton, Willesden Jn & Clapham Jn, which I doubt is the "best" option by any definition) between Rickmansworth & East Croydon, but then you have to go to the gateline and tap out to continue on a paper fare.
The lowest priced paper fare from East Croydon to Brighton is a "Thameslink Only" Off Peak Day Return at £10.10. According to the terms of the Ticketing Settlement Agreement, this is valid on the train company of that name, but in the event of a ticket check on one of the trains operated by Govia Thameslink which is branded "Southern", the on-board staff might disagree and attempt to charge you more money.
For £13.15 you can get an "Any Permitted" fare which Govia Thameslink staff won't falsely claim is not valid on all trains they operate.
Can a boundary ticket be ordered online so I can collect from a TVM?
No. You can purchase one at almost any GTR operated TVM though, using the alternative origins option.