Yes, ext_flag is (currently) only 0 for key path spends. The only defined script path spend (Tapscript, BIP342) sets ext_flag to 1.
The size of a Taproot ScriptPubKey is indeed always 35 bytes because there is:
- 1 byte for the size of the size of the script
- 1
OP_1 - A 32 bytes push (which is 33 bytes accounting for the length prefix)