Which could prove entirely wrong, of course.
Ukraine seaports are useless right now. Ukraine’s or international ships can’t break through the Russian naval blockade. The only exit for Ukrainian wheat, for instance, passes through Ismail city, a port on the Danube from where it is carried away by other than Ukraine’s ships up on the Danube or to the Black Sea.
To sever this last naval connection Russia does not need to occupy Odessa first, it is easier to push the Kherson bridgehead towards Transnistria and cut the Odessa region from the rest of Ukraine.
Ukraine invading Republic of Moldova (Transnistria is officially part of the Moldovan Republic) enables Moldovan government to ask for help. Since the bulk of Moldova’s population (disregarding Transnistria) is Romanian and since Moldova’s territory (again minus Transnistria) is a former province of Romania, the natural support should come from Romania.
I don’t know if there are Romanian troops ‘in disguise’ in Moldova. If they are, they are not there to fight posing as Moldova’s troops. The purpose would not be to fight against the Russians alongside Ukraine, the purpose would be to secure Moldova (minus Transnistria) in the Western sphere of influence, most probably after a referendum which will integrate it back into the Romanian state. The Crimean precedent set by the Russian soldiers (‘little green men’) could serve as a guideline. It is also very possible they will display their true identity the moment Moldova officially asks for support and restrict their deployment exclusively at the Nistru/Dniestr river.
Why would Ukraine act as a pawn, practically disregarding its own interests ? For the same reason it is constantly acting like that for the last 8 years.
Russia might not be delighted but it is a satisfactory outcome for its geopolitical designs. An Ukrainian attack on Transnistria offers them the needed justification to reach that area and incorporate it. Which would seal the fate for the prize they desire most – Odessa.
The West might not be delighted either but, realistically, both Russia and the West (read US) can extend so much before reaching their limits. The scenario I sketched here fits reasonably well most interests. Except Ukraine’s, who’s defunct anyhow.