Questions Downlink OFDMA in LTE
In LTE downlink, users are multiplexed in both time and frequency domain. The concept of ressource blocks (RB) describes a certain frequency band in a certain time slot. Your understanding of the steps in the transmitter are correct. Step 4 describes the procedure that assigns a certain frequency band $B_m$ to some user $m$. One OFDM symbol in LTE downlink carries the data of several users. Subcarrier mapping means that the data of user $m$ is allocated to certain subcarriers that represent the frequency band $B_m$ as shown in the following figure (where $M$ is the total number of frequency bands)



This technique corresponds to frequency multiplexing. Time multiplexing means that the the band $B_m$ is reserved for user $m$ only for a certain time duration, i.e. for a certain number of consecutive OFDM symbols (you mention 14 in your question, I haven't checked it). In the next subframe this band might be allocated to a different user.

The bit rate of user $m$ can be calculated as follows. Let $N$ be the number of subcarriers per user, $b$ the number of bits per subcarrier and $T$ the OFDM symbol time including the cyclic prefix. Then the bit rate $R_\mathrm{b}$ of user $m$ is
$$
R_\mathrm{b} = \frac{bN}{T}
$$