Can I Replace An Old Wallet.dat File In Bitcoin Core Before It Fully Syncs?
I assume you are cleanly shutting down bitcoin-core before you overwrite wallet.dat.  This is absolutely a requirement.  You can't replace an open wallet file while bitcoin core is running.
#1 - Yes, you need to complete the initial block download to see the contents.  Since you already download some blocks before the wallet born-on date, you will need to issue a rescan to look for wallet transactions before the block your currently downloading
#2 - You can always see the keys of the wallet regardless of how far the download has gotten, but you won't get the balance until the download is complete.  Once you have all the blocks you can do everything offline, but of course it won't see TXNs in blocks that you haven't grabbed since going offline.