Clarification required on DALI Standard 101,103
Q1: Your product needs to comply with Parts 101 System and 103 Control Device. But you need to read and understand Parts 102 Control Gear and 209 Color Control so that you know the commands that are available - the format of those commands, the use of DTRs, the requirements to send twice and/or to wait for responses etc.

Q2: If you don't use the commands listed in 209 for colour control, you won't be able to control commercial control gear which implements 209. You cannot use reserved commands and pass the compliance tests, but there are ways of communicating on a DALI bus which are allowable manufacturer specific methods such proprietary frame size (number of bits or bit timings) and Operating Modes.

Q3: see answer to Q1.

The cost buying standards can be reduced by joining the national standards body, but this has its own cost so you would have to decide if it was worthwhile. The cost of the standards is a small fraction of the cost of developing and certifying a DALI product - if you want to use the DALI name or logo on the product, you also have to join the DiiA and pass the official test sequences. These are to ensure that all products are interoperable.

Edit: if you are not interested in compliance at this stage but just want to get some functionality working, then part 103 is the most disposable part because you are only implementing a system which has a single control device on the bus, and part 103 is mainly about different control devices communicating with each other.

The various micro manufacturers NXP TI ST MCP have DALI App Notes which often include (usually out of date) information which covers the essentials of part 101 and 102, so that might be sufficient to get started. However, you are less likely to find any coverage of part 209 outside of the standard, so you probably need to purchase that one at least.