Non-mining-related nodes don't matter. Transactions will reach mining nodes/pools. Non-mining nodes are only interesting for its owner, to see the block chain for himself, and verify its validity without having to trust another full node. But full nodes that do not relate to mining don't matter in the consensus mechanism.
What does matter, are users. They 'vote with their money' and determine market cap.
you have been sold alot of BS
1. exchanges are the nodes. if they orphan block X.. your TX in that block doesnt exist = you cant spend because they dont see it
2. if a friend is using a node and you pay that friend. if they orphan block X.. your tx in that block doesnt exist = you cant spend because they dont see it
You are talking about USERS, not "nodes". Again, we have to distinguish several cases. Suppose that we have a full hard fork (not backwards compatible). If my friend, or my exchange, is using a "node only compatible with variant X", then it simply means they are only accepting COIN X. If I try to pay them with coin Y, it won't work of course. That's like as if I tried to pay a bitcoin user with Ethereum. So the full hard fork case is easy: we're simply talking about different COINS.
Now, suppose that we talk about a soft fork. If the soft fork has a majority amongst miners, that soft fork is simply imposed upon all miners. Simply because every non-soft forked miner will always produce orphaned blocks, because they are considered invalid by the majority of miners. So his only chance of getting accepted blocks on the longest chain, is also to apply the soft fork conditions. The users nor the nodes have anything to say about it, because there's only ONE CHAIN. They accept it, or they come to a grinding halt. A soft fork with a majority of miners is IMPOSED upon everybody: the minority miners, the users, and the nodes. Nodes with both protocols will accept the chain, and it *doesn't matter* what protocol they run.
Now, suppose that the soft fork is in a minority amongst miners. That means that the majority of miners will regularly put blocks on the chain that are not accepted by the soft fork, but this will be *the only chain* that is available. If your node is running soft-forked software, then it will simply REJECT the sole chain that exists. But no acceptable chain will be around. Because majority-non-soft-forked miners will still add blocks to the longest chain, and the "pure soft fork chain" will not exist. So your node COMES TO A GRINDING HALT if it is soft-forked.
The case of the backward-compatible hard fork looks more involved. As long as the hard fork has a majority of mining hash rate, BOTH COINS exist. If you want to pay a friend in "hard forked coin", you have to send "hard forked coin". It is ANOTHER coin (an alt coin), and of course if you try to send him your original coin, it will not work. Idem for an exchange. A hard fork that remains a majority is similar to a full hard fork: TWO COINS exist now with separate block chains, transactions, wallets and everything. If an exchange, a user, .... only decides to use ONE of the coins, then you have to use THAT COIN to pay him.
It becomes a major cluster fuck if the hard fork that had majority, loses this majority (determined by MARKET CAP, and miners that will follow to optimize their block rewards). After a while, the old chain will become the longest one, and because the hard fork is backward compatible, the former hard forked chain STOPS and is orphaned. Every wallet there is then simply LOST, and all transactions on that chain are REVERSED.