1. Depends on alternators, amplifiers and ohm load the amplifiers will be wired to.
2. Yes just like AGM they will increase in amperage output as voltage drops. But they start discharging from the ~14v range versus the mid 12 range like AGM, so they have the potential to hold high 12's/low 13's for a lot of people.
3. They will be fine to mix. The AGM's resting at a lower voltage will draw from the lithium, which is OK. Lithium prefers to be stored at lower SOC, contrary to AGM which basically higher = better. Any charger will suffice really.
4. Lithium is extremely worth it, and is actually the cheapest option (by a good margin) if you can afford the higher up-front cost. But with JY releasing their 8.55AH single module 12v cells in a few months at roughly $240 shipped each it will make it easier for people to break into the lithium market. People seem to be happy with JY and Limitless. Limitless does have some con's though. The non-modular design makes it impossible to switch to/from 12/16v without replacing the entire battery. The JY's you will only need to add/take away one cell. Limitless doesn't have passive cell balancing either. There is a .5A draw in each JY module that will bleed current until a certain voltage set-point. This indirectly balances all cells with each other without manually balancing them or using a BMS.
Also a sidenote, not a single cell has failed since JY's inception. Have heard of numerous cases with people having issues with their XS batteries or being very let down by the performance. Scottie seemed super interested in sending me a lithium to include in my testing until I posted the Winston Vs JY video.... Odd.