1. Does delaying the Qadha invalidates it(without valid excuse)? 2. Are we allowed to trust the students of renowned scholars of their sayings until we see major falsehood without repentance? 3. Is it haram for me to tie a bell on my cat so she doesn't get lost? I don't put it inside the house.

1. No. Delaying qadhāʾ does not invalidate it. Delaying without a valid excuse is sinful, but the obligation never lapses: it remains a debt owed until performed. For Ramadan fasts delayed past the next Ramadan, the majority of scholars added feeding a poor person per day alongside the qadhāʾ, while others held the qadhāʾ alone suffices — giving it as a precaution is good. 2. Yes, when the student is recommended by the scholars and his knowledge also testifies to this. Ibn Sīrīn raḥimahullāh said: "This knowledge is religion, so look to whom you take your religion from" (recorded by Muslim in the introduction to his Ṣaḥīḥ). However, we can not just recklessly judge people as well. We have to make sure first before making judgements and return to the scholars as well before that. 3. Permissible. Shaykh Ibn ʿUthaymīn raḥimahullāh explained that the prohibited bell — "the angels do not accompany a group in which there is a bell" (Muslim) — is the one whose sound is sought for amusement; a bell serving a real purpose for an animal carries no harm, though leaving it is better if something else does the job.