a) Is it realistically possible to map the tx graph on Monero if receivers are not visible? (unless you are using Monero on exchanges or have the private keys to a wallet) Every hop makes it exponentially more difficult to find a sender. After only one hop away from the original transaction there is less than one percent chance of guessing sender correctly (~0.4%), two hops (~0.02%) etc. (not even considering you could be chosen as a decoy for someone else too) But if this worries you I'm not sure why you would use Bitcoin as mapping a tx graph is 100x easier to do.

Also, if this is still your concern, Seraphis upgrade will bring FCMPs (Full chain membership proofs) that are replacing ring sigs. They are being worked on already. Every transaction will eventually be a potential member of every transaction that has ever occurred with FCMPs.

b) Fair concern, but Monero cryptographic commitments are based on relatively old tech from the 80s. Pretty well established and tested so invisible inflation is in realm of possibility but unlikely. If it still concerns someone you can just sidestep this by only treating Monero as a checkings account (having a small pool for transacting) and using Bitcoin to save until Bitcoin has better privacy tech (if ever?)