They can always loan one out, there will be pretty much always be demand at other railways for steam locomotives.
There will, but there's a limited number of lines who can make use of a heavy freight 2-8-0. Most places need nothing bigger than an Austerity, GWR Prairie or a Standard 4.
Wensleydale polar express…
I doubt this for two reasons - firstly and less significantly, its arguably the 'wrong' colour. The average punter is far more likely to be fooled by any black tender engine dressed up as 1225 - a grey engine, even if American rather than British, will 'look' wrong and contrasts with the image they've built in recent years. This isn't a major thing though, as the likes of the Spa Valley running a green Bulleid Pacific have proved.
More significantly though, to transport 5820 from Keighley to Leeming Bar would mean hiring two lorries for road transport. Wensleydale have an established relationship with Rileys to bring in a Black 5 by rail each year - unless a 5MT wasn't available I can't see why they'd change the plan.
I haven't forgotten that they did use one of the CVR's S160s for the first few years though.
Wensleydale would probably appreciate one over the summer too.
There are quite a few other candidates too- Ecclesbourne Valley or even the North Yorkshire Moors could probably benefit from another powerful loco. Given the seemingly positive working relationship with the GCR, one could end up there too.
Wensleydale have been open about the fact that they don't currently have the capacity to host a Steam loco long-term though they're currently working on this so that may well change in the coming couple of years. The NYMR, if anything, desperately need additional motive power that can work through to Whitby rather than internal services - don't forget they've got the 9F and S160 Omaha and the Q6 which are all Pickering - Whitby only.
GCR likewise are fairly well covered for heavy locos I think? EVR not impossible but their staple is industrial tank engines rather than larger engines.
Fully accept that I could well be proved wrong. But I can't see KWVR overhauling Big Jim purely on the basis of being able to hire it out.