The real purpose and advantage of having a shadow cabinet is to show the electorates that you party is ready to form a government when needed. The members of the shadow cabinet will be subjected to scrutiny from the public on the suitability for the ministerial role - their performance in debates, relevant work experiences, etc will be examined.
Trying to force the formation of a shadow cabinet now will turn out to be a big PR disaster for WP. Afterall, Mr Low Thia Khiang himself recently said that the WP is not ready to form a government. Having a shadow cabinet will pretty much contradict what he has said. The current MPs that WP have in parliament are also not suitable for the shadow cabinet roles available.
Some patience is needed here. WP will eventually form a shadow cabinet when it becomes strong enough to be able to form an alternative government. To get there, WP will need to attract candidates with good experience with public policy. Former PS or PPS like Tan Jee Say and Benjamin Pwee, minus the ego, would be great. Then stick them into GRCs that the WP are looking to win from the PAP.