In case a node doesn't verifies signatures we don't have to consider it as a full node i.e. we can't rely on such node's decisions because they will accept correct PoW with wrong signature. We have to consider such nodes as light wallets.
Exactly, yes, that's why I called them a "lightweight client model"

Network with majority of nodes not verifying signatures can be easily manipulated by big pools.
Full nodes will still verify signatures (but as I mentioned earlier this is not critical path).
Let's calculate scratchpad reduction benefit for a low-end device in case of current block verification:
- 120 seconds block target time
- 10 h/s hash calculation speed
In this case one block verification takes 0.083333% of one CPU core time. Is this a showstopper for mobile device?
It could be. If you are two days behind when you open your wallet, that is 144 seconds to sync. Quite a long wait.
Also, I'm not sure that 10 h/s is achievable on a mobile device with scratchpad in RAM. Maybe, but that is aggressive. It is more realistic if scratchpad is in cache.