If you look at the map, you will see that there is a line of workings from Old Wheal Jane, through West Jane (new jane) and Nangiles, then through Wellington into United and on through Squire and Ting Tang.
There is a parallel line of workings through Consols (including Maid) through Carharrack mine and onwards towards Wheal Damsel.....(and several other parallel series of lodes).
Mount Wellington went out under United, but not to a massive extent and most certainly not on the exact main United Lode.
In the 80's the Wheal Maid decline was sunk from the area between Wheal Maid and West Wheal Fortune (both Consols) and this dipped at about 1:7 towards the parallel United lodes. It was thought that there was a lot of tin in the small Whiteworks Mine (which there is) and that the United Miners were in such a rush to remove the copper bonanza, they left parallel lodes standing and any tin associated with the wall rock. They were going firstly through a gap in the stoping into United (which they never got around to) and then another branch off under Ting-Tang and the Damsels. They never got around to this either.
In conjunction, they moved their mobile headframe around various shafts in the area (which are noted by their nice collars) and did some very nice plans and measurements.
In my opinion, the Gwennap district is the most fascinating of all in Cornish mining.
The decline is well buried and goes straight to water. If you had diving gear, you could come up in Whiteworks which was buried by the nice local scrap man. Interesting hole with lots of tin in, I gather. Jon Peck did a book called "Wheal Jane Underground" which is worth a read, as is the "Painting a Mine with Light" and the rest of it is poorly recorded, although there are lots of people in the area who you can talk to who were involved with the workings.
To your original question, United and Old Jane are connected by workings which you could walk from one end to the other from approx Frederick's Shaft of United to Tippets East of Old Jane.
There are some plans floating around on Mine Explorer, if you look for them.
The main reason for doing this work was that Poldory in United and Whiteworks were looked at in WW2 and got encouraging values. This is recorded by Dines. The work was re-investigated by the then Wellington Company. Sadly, Whiteworks is under Drew's yard and is gone for good and Poldory is under the dump.
I've gone in most of it, apart from the bit immediately under the tip, which would be very interesting, but probably deadly. I gather there are still some things like wagons on rails in Poldory....all sadly buried under the tip.
If it had been down to me, I would have made a rubbish mountain on the outskirts of Newquay instead....improved it a bit.