With such a numerical decline in the community it now makes sense for regiments to submit a team roster with player GUIDs. The league should be divided up into 4-5 stages (a stage consisting of 2-3 matches). At the end of each stage there should be a 2-3 day period where all players that wish to join new teams can (this includes external players wishing to join teams that are in the tournament). These changes should be listed on a post at the end of each stage. Whitelist the servers so that anyone who has not complied with the policy will simply be banned. In the past people have criticised this approach stating that it is too much effort but frankly every single RGL since season 2 has been plagued by the problem of double reggers so putting a little more time and effort in to construct watertight format is in the best interests of the organisers and the regiments.
The second point regarding people being in two regiments, one casual and one competitive, is virtually impossible to prevent. One can only manage the members that are in the RGL but there is no way of finding out what other regiments they are in if they really want that to remain hidden. A policy like that which the EIC introduced where no double regging of any form was allowed could be a start but it was hard to enforce then and it will be hard to enforce now.
Also, for once, can the organisers set up a criteria for accepting regiments rather than just going with whoever signs up. It is better to have a smaller roster of participating regiments who you know will stay till the end than have a larger roster which is half empty by the end of the season due to unstable regiments dropping.