Rather than go through that again, I'd prefer we use the
backwards compatible proposal from BIPs PR#945 and, if we want to
maximize safety, consensus restrict v1 witness program size, e.g. reject
transactions with scriptPubKeys paying v1 witness programs that aren't
exactly 32 bytes.
Adding some kind of relay policy rule would be easier than a consensus rule, and maybe effective enough. (This comment is not intended to endorse any one proposal over another.)
Hopefully by the time we want to use segwit v2, most software will have
implemented length limits and so we won't need any additional consensus
restrictions from then on forward.