Sorry to hear about your issues with INAV. Over the past month or so I have been reading up on INAV because I am a little frustrated with the direction BF is going now. I will probably still need to upgrade BF sooner or later but for now I am happy with my quad on BF3.5 

i have BF RTH and Failsafe running great. I test them EVERY time i fly ( i probably have 100+ RTH/Failsafe checks this summer) and while I dont let it come down any closer than about 25' to me before I take control back. But I really thought that having the other features of INAV and the ability for it to land would be way cool.
INAV does interest me a lot. So this winter i will be setting up 2 quads on INAV and upgrade to BF 4.1 on the other. 

As I was reading your post I was thinking...too many changes all at once. I am like wingspinner: get the basics down pat and go fly a few packs.  Add something else and go fly a few more packs. That way if it misbehaves you know where to aim the hammer .

And i also agree...for me F7 and baro's

Hope you get going without too much trouble and money.