Everyone connected to that full node with an SPV wallet is trusting that node. They are trusting that that node is giving the correct information.
Yes I don't dispute this. But what is unclear is if adding additional "network only" nodes (is "relay nodes" a better term?) improves that situation.
For example how do SPV wallets determine what nodes they get information from? Do they always connect to the same one? Is it random and they get messages from many nodes (from the whole network??)? If they get messages from many nodes then having more network nodes (and assuming the majority are honest nodes) somewhat increases the security of SPV wallets.
If you have more info on how SPV wallets get their info (typically from one node or whole network?) then that might help. I'm not that familiar with SPV wallet protocol.
It depends on the SPV wallet as they use different methods of getting their data. Most of them will connect directly to nodes just as a normal full node does, except they don't relay, store, or validate blocks and transactions. These nodes will connect to the network like a full node, so their connections are not always the same and having a diversity of nodes makes it more secure as it is less likely that all of its connections are colluding to provide false information.