The main motivation cited is reducing block orphaning
Where do you see this being cited?
AFAIK, the motivation is to reduce the resources required to run a full node which has nothing to do with reducing block orphaning. Furthermore, the size of the UTXO set does not really effect block orphaning that much, especially with compact blocks and FIBRE allowing blocks to be relayed without being fully validated.
My personal fullnode does not care about if the UTXO is fetched from disk and takes time to verify. (as long it's well under 10 mins)
Your node does not just need the UTXO set when verifying blocks. It constantly needs it to verify the transactions that it receives. Your node does not just sit idly doing nothing between blocks, it constantly receives transactions, verifies them, and relays them. If access to the UTXO set is slow, then verifying these transactions is also slow. It also means that your computer may be devoting a lot of extra resources in order to try to process transactions quickly which will impact its performance in other tasks if you use your computer for more than just a Bitcoin full node.
1) Is there some other reason for research behind UTXO set size other than block-orphaning?
As mentioned earlier, block orphaning is really unrelated. The main reason is to keep the cost of running a full node down to allow full nodes to be run on as low end hardware as possible (e.g. a raspberry pi) so that there are more nodes. Having more full nodes is better for the network, and keeping the size of the UTXO set small helps allow for lower powered hardware to be used for full nodes.
2) Directly related to the first question, is the UTXO set size really that important for non-mining full nodes?
Yes. All full nodes need the UTXO set in order to validate transactions. If the UTXO set is too large, it becomes more expensive to retrieve data from the UTXO set which thus increases the costs of running a full node.
Presumably you do other things on the computer that runs your full node. You don't want the node to take up all of the resources of your computer, otherwise you wouldn't be able to use it to do other stuff. Part of keeping the node performant is to reduce the size of the UTXO set.