Are the Heaven's Feel movies accessible to those who've never watched any Fate/* stuff before?
It's one of those series that feels a little intimidating to get into, considering how many releases there are and how convoluted it seems to be overall.
As others said, Heaven's Feel is probably the worst starting point given it skips over early content from UBW S1 in an intro AMV thing. At very least, you need to have watched S1 UBW. S2 best skipped cause it's shite, on-semi-related note. HF1 rather oddly spends an hour showing Sakura scenes since she was barely in the other anime routes. First film only feels like it gets started after that, so I recommend starting with UBW S1. Zero is a lot better than UBW bar the amount of waffle vs deaths in S1, though. One a novel adaptation, the other a smut novel, and difference in writing shows. I'd say it only spoils thing most would see as minor with fanservice and cameos. But, eh: I've always been a more casual anime-onry viewer.
I went DEEN Saber route 4/10 > DEEN 5/10 UBW movie > Zero > UBW > Heaven Feel. Mainly cause that the order they released.
PS: Anyone that refers to Saber as 'Seiba' is a filthy mongrel. This is an English message board, have some class. You aren't appealing to your Japanese masters 'ere.