Masbooq (latecomer) joining the Imaam before Salaam

Q: Whilst performing Salaah with Jamaat behind an Imaam, if a Masbooq (latecomer) joins the Salaah after the Imaam has completed four rakaats and joins just before the Salaam.

1. Is it necessary for him to recite the tashahhud with the Imaam for his Salaah to be valid or is it sufficient for his Salaah to be valid that he joins in the Salaah before the Imaam makes salaam?

2. Should the Masbooq recite only Tashahud in the Qa'dah or also Durood Ibraaheem and Duaa after Durood.

Experiencing a discharge due to sexual thoughts

Q: I was at friends house and some of her guests were saying inappropriate (sexual) things and I was trying not to listen but when I did, I got weird feeling in my private area and like something should be put up in the area (private area) but I didn't. I just sat there till the feeling left and when I came home, I went to the washroom and I had a bit more discharge than usual like it was kind of a lot. Should I do ghusul?