Can returns completed and filed using Desktop be accessible in Documents on Web? My July 2019 filings aren't (I had to switch to Desktop mid filing but usually use Web)

No.  Online accounts and desktop program is different.  In the Desktop program your return is only saved and stored on your computer.   Be sure to save your Desktop return as both the .tax2019 file and pdf files.  Next year you will need the .tax2019 file on your computer to transfer into 2020.  You can transfer it into either the Online version or the Desktop program.  

Or did you mean your 2018 return you did last July 2019?  Do you need  to start 2019 return with it?  If you have the .tax2018 file on your computer you can transfer it into a new 2019 online return.  If you already started 2019 you will need to clear and start over or set up a new online account.  

How to Transfer to the Online version

https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-turbotax-online/00/25953