I went ahead with the direct naval invasion of Petrograd, fortunately, my naval force arrived on day 13 so the invasion force landed the next turn, but I was able to have better intelligence as to what forces were where and I decided it was worth the risk.
My forces landed and was able to get a foothold and a breech in Petrograd so an assault will happen the next turn (a capture of a supply wagon didn't hurt either). The enemy force wasn't crushed, but retreated to a nearby province. I will bring in more reinforcements just to be sure, but this strategy does seem to be working.
The Revolution occurred during the invasion so with the soon to be played Kornikov decision, the Russians will hopefully be out of the war within a month or two. It is now May '17. It can't happen fast enough for me as the WE had a rare successful assault on my besieging forces in Compeigne (next to Paris). I can definitely start bringing more troops to the Western Front, as gaining more strategic cities in Russia should no longer be necessary. On the plus side, Belfort and Epinal now look less well-defended as the WE has moved some troops away to help protect Lyons and their recruiting grounds and a city next to it which has an armory and munitions plants (or something like that) thanks to successive captures of two cities that are on the rail to these important cities. Or I may just use most of these forces to reinforce my cities near Paris. The options look pretty good on the Western Front. In Egypt, the Brits were routed in Cairo and Damietta, so the rail to Alexandria is now free of enemy troops! Hopefully, the war can end before the presumably large American forces enter the scene (as of yet, the American entry in Dec '16 has had almost no effect on the WE PWR rating; in fact, it is now down to 67/188 from their pre-entry PWR of 70/175 and that is with Romanian entry into the WE)!