Culra bothy is ‘officially’ closed due to asbestos. There is some chat about rebuilding.
GL descriptions of the routes in are about right. From Rannoch you’ll get to the Loch end by rough track and then it is portage up the loch side. From Corrour you get forest track to the hydro scheme at the lodge then up the Glen with bad portage to nip over Bealach Cumhann. The descent from there would be okay on a MTB but the rest is somewhere between crap and awful.
In summary why bother when there are bothies so much better suited to approach by tourer or even MTB? E.g. Ruigh Aitchechan, Bob Scotts, Glen Dubh Lighe, Invermallie, Glen Buck, Luib Connel, Inshriach, Ryvoan, Glen Dhu, Corryhully, Rackwick, Resourie, Kettleton Byre, Melgarve (I could go on).
(edit – bold ones might be linkable into a wee tour now I think about it)