Sorry, but there is a saying that when in a hole, you should stop digging. Your two previous posts reinforce what I've said - you can claim what you like, what you demonstrate is your lack of knowledge in the area.
If your cabling isn't gigabit capable then the only logical reason for that is you followed the false advice of someone on the internet (like yourself) who told you not to bother installing it properly. If you installed Cat5e properly, then it'll handle gigabit. You can pick up a gigabit switch now for peanuts - about what 100Mbit switches cost a few years ago. So as soon as two of your devices have gigabit then you gain.
The internet connection is largely irrelevant - though it's not actually all that far off now - FTTC goes up to 80M, FTTP goes up to (IIRC) 300M at a price. Lots of internal devices gain from gigabit vs running that at 100M - having a NAS (or other form of server) is quite common now and that really gains, as do my backups which take an age over 100M. Even the humble WiFi access point can gain now - some have (claimed) wireless rates well in excess of 100M.
I can tell you that copying files across the work network (100M) seems reeeeaaaaalllllyyyyyy ssssllllooooowwwwww compared with copying stuff at home where most of my kit is Gbit.
And you CAN mix and match - having a 100M switch won't reduce your gigabit switch throughput, it'll only reduce the throughput of traffic that actually goes through a 100M link/switch. SO there's no problem whatsoever having a 100M switch supporting oder kit (one of my printers is only 10M !)
The simple answer to the OPs post is : don't bother planning on fudging things, just install to the standard. If you do that then anything will "just work". You won't be coming back at some point in the future moaning that you didn't pull two cables when it would have been easy, and can anyone see a way around your problem
Ignore idiots you tell you to not bother with the specs. They are shortsighted and will just lead you into making bad choices.
I understand all that you said and you can keep waffling on !!!!!!!!