Metal-organic frameworks (MOFs) are well-known porous materials owing to their useful adsorption properties; however, some MOFs have limited adsorption capabilities, which can significantly undermine their success as porous materials. Therefore, maximizing their porosity is critical for unlocking their full potential and expanding their practical utilization, such as gas storage, separation, and removal. In this study, flexible MOFs with defined defects were synthesized using a ligand-mixing strategy to improve their porosity and maximize their adsorption capacities. Specifically, we employed a combination of two organic linkers, 4,4'-biphenyldicarboxylic acid (H<sub>2</sub>BPDC) and 1,4-benzenedicarboxylic acid (H<sub>2</sub>BDC), in various ratios, to fabricate flexible In-MIL-53D hybrids containing controllable defects within the structure due to the incorporation of the short linker (H<sub>2</sub>BDC) compared to the original linker (H<sub>2</sub>BPDC). These structural defects in the In-MIL-53D hybrids activated their gate-openings and enhanced gas adsorption capacities for N<sub>2</sub> and CO<sub>2</sub>. Moreover, the gate-opened activated hybrids exhibited excellent adsorption capacity for the harmful chemical warfare agent simulant, 2-chloroethyl ethyl sulfide (CEES). However, excessive incorporation of defects disrupted the framework's integrity, compromising its stability and increasing the risk of collapse. Therefore, achieving an optimal level of defect incorporation is essential to balance structural stability with enhanced functionality. Among the hybrids, the sample with approximately 39% incorporation of the short linker exhibited up to an 11-fold increase in adsorption capacity for CO<sub>2</sub> at 1 <i>P</i>/<i>P</i><sub>0</sub>. In addition, this hybrid demonstrated up to 5-fold higher CEES adsorption capacity compared to the pristine In-MIL-53D, highlighting its potential for advanced utilization in relevant fields.